In many episodes of Star Trek TOS, Kirk is seen either drinking coffee or being served coffee by one of his yeomen. With all that is known about Kirk in Memory Alpha and other canonical sources, it seems strange to me that I do not know what kind of coffee he drinks. After all, Captain Picard is known to specifically drink Earl Grey tea. Does Kirk have a known caffeine preference? Is it possible that it depends on which yeoman is serving him? This would make some sense, as his behavior on the bridge can range from the very animated to the slightly sleepy.
In the Trek novel Enterprise: The First Adventure a bleary-eyed Kirk snaps at Yeoman Rand before heading for the synthesizer to get the day's first coffee. The coffee tasted awful but Kirk drank it anyway and then woke up sufficiently to realize he'd been rude to Rand and apologized. No one drinks bad coffee first thing in the morning unless it has caffeine in it, so I'd say that Kirk drinks caffeinated coffee.