As a converse to this question, what's the first clear evidence in the Harry Potter books of Hermione knowing/realising she was romantically attracted to Ron?
I'm guessing it's after the Yule Ball and Krum, and 6th year at the very latest. I'm also interested in whether it's before or after Ron realised his feelings for her.
I mentioned it on some comments before, but I think it can make an answer of its own.
I would say that a particular affection was hinted as far back as GoF and the whole Yule Ball thing. But nevertheless it might back then have been something that the characters themselves did not realise. Note that Hermione was somehow expecting Ron to ask her to the ball. Not Harry, with whom she had a closer friendship. And indeed it was Ron who did, albeit too late.
In GoF, the whole Hermione-Krum part is left somehow cloudy in the books. We don't know for certain what has or hasn't happened, but when Ginny replied back at her brother in HBP, she clearly indicates that Hermione was with Krum. Same when later she invites McLaggen, the other Gryffindor girl also mentions that. So we don't know for sure, but there is a possibility that indeed something happened. This is actually more shown in the films with the discussion Harry and Hermione have about it on the Bridge.
Nevertheless the relationship she had (or didn't have) with Krum seemed to be a marker for Hermione to realise the growing feelings she had for Ron.
Interestingly, she was somewhat supportive of Harry's relationship with Cho. But not at all of Ron's with Lavender.
HBP and the whole Lavender thing seems to be a strong indication for me and would count as the first clear sign. When Lavender kissed Ron, somewhat surprisingly for him, she left the place and Harry followed her. She was upset about it. There is the McLaggen thing, where she tried to make Ron jealous. And finally, the hospital scene, when Lavender left Ron, and Hermione stays with him.
Earlier in HBP, she identified the smell of Slughorn's love potion. And she hushed the last one, making it plainly clear that she was conscious of having feelings for someone, later understood to be Ron as e.g.:
Jess Mac: What was the third smell that Hermione smelt in the Amortentia potion in HBP (ie the particular essence of Ron)?
J.K. Rowling: I think it was his hair. Every individual has very distinctive-smelling hair, don't you find?
Some points which might be of interest from DH:
I don't have the books in paper form, so any quoting is a pain, but I tried to be explicit enough with the scenes.
As far as "clear evidence" is concerned, the first overt sign in the novels that she's attracted to Ron is when she actually kisses him in Deathly Hallows. Prior to this they simply bicker a lot and blush occasionally:
There was a clatter as the Basilisk fangs cascaded out of Hermione’s arms. Running at Ron, she flung them around his neck and kissed him full on the mouth. Ron threw away the fangs and broomstick he was holding and responded with such enthusiasm that he lifted Hermione off her feet.
‘Is this the moment?’ Harry asked weakly, and when nothing happened except that Ron and Hermione gripped each other still more firmly and swayed on the spot, he raised his voice. ‘OI! There’s a war going on here!’
As to why she finally acted on her attraction to him, in true Hermione style it seems to be when Ron used his brain for a change:
‘But how did you get in there?’ he asked, staring from the fangs to Ron. ‘You need to speak Parseltongue!’
‘He did!’ whispered Hermione. ‘Show him, Ron!’
Ron made a horrible, strangled hissing noise.
‘It’s what you did to open the locket,’ he told Harry apologetically. ‘I had to have a few goes to get it right, but,’ he shrugged modestly, ‘we got there in the end.’
‘He was amazing!’ said Hermione. ‘Amazing!’
And then shortly afterwards when he shows genuine interest in the wellbeing of the House Elves:
‘The house-elves, they’ll all be down in the kitchen, won’t they?’ ‘You mean we ought to get them fighting?’ asked Harry. ‘No,’ said Ron seriously, ‘I mean we should tell them to get out. We don’t want any more Dobbys, do we? We can’t order them to die for us –’
There was a clatter as the Basilisk fangs cascaded out of Hermione’s arms. Running at Ron, she flung them around his neck and kissed him full on the mouth. Ron threw away the fangs and broomstick he was holding and responded with such enthusiasm that he lifted Hermione off her feet.
I think their chat when Hermione refuses to go to the Yule Ball might show that Hermione has realized her feelings. Quoting Harry Potter and the Goblet of Fire chapter 22.
‘I can't come with you,’ said Hermione, now blushing, ‘because I'm already going with someone.’
‘No, you're not!’ said Ron. ‘You just said that to get rid of Neville.’
‘Oh, did I?’ said Hermione, and her eyes flashed dangerously. ‘Just because it's taken you three years to notice, Ron, doesn't mean no one else has spotted I'm a girl!’
Ron stared at her. Then he grinned again.
‘OK, OK, we know you're a girl,’ he said. ‘That do? Will you come now?’
‘I've already told you!’ Hermione said, very angrily. ‘I'm going with someone else!’
And she stormed off towards the girls' dormitories again.
As an additional clue, J. K. Rowling's old website contains the following FAQ and terse answer.
Does Hermione love Ron or Harry?
I can't believe that some of you haven't worked this one out yet, but I'm not going to answer because that would spoil the arguments, which I enjoy.
This has appeared on the website before 2006-02.
I take this as Rowling claiming that it should already be obvious to at least careful readers of the first six volumes who Hermione loves. I think it also implies that Hermione has realized her feelings by the time of the end of Half-Blood Prince or earlier.
I'm gonna make a case for sometime during her fifth year. I would point to the discussion between Hermione and Harry about his first date with Cho:
'You should have said it was really annoying, but I'd made you promise to come along to the Three Broomsitcks, and you really didn't want to go, you'd much rather spend the whole day with her, but unfortunately you thought you really ought to meet me and would she please, please come along with you and hopefully you'd be able to get away more quickly. And it might have been a good idea to mention how ugly you think I am, too,' Hermione added as an afterthought.
'But I don't think you're ugly,' said Harry, bemused.
Hermione laughed.
'Harry, you're worse than Ron ... well, no, you're not,' she sighed.
Harry Potter and the Order of the Phoenix - p.505 - Bloomsbury - Chapter 26, Seen and Unforeseen
As evidence that Hermione knows she was romantically attracted to Ron.
I've read through all the answers and I know some people may have the bar slightly higher for what counts as 'romantic feelings', but I think this quote demonstrates that Hermione is very much interested in Ron and she knows it and she is consciously sad that he's so difficult in that arena.
I also know that some people may have the bar slightly higher for 'clear' evidence, but I see this (personally) as an explicitly sad reflection on her relationship (in the broadest sense of the word) with Ron.
But you may think I'm reanalysing the conversation after the fact. I think it's worth considering though. In my opinion, it's the first time we see Hermione show herself to be sad about Ron's interpersonal skills and I read this as a woman reflecting sadly on how she wishes things were a little different. Given the context - right after Harry's date - I don't think this is merely about his tactlessness in friendship.
I believe she revealed them in the 5th book (the one I am on now), because before a Quidditch match she kisses him on the cheek. And in the 6th book she is jealous of Lavender Brown.
Hermione never had any problems with Harry going out with girls and kissing them. But she became upset the moment she saw Ron kissing Lavender after First Quidditch match in HBP. She goes to empty class room to avoid that scene. I think Hermione is clever enough decipher these feeling without any help on her own quickly.
