not much, caffine may even help uptake, at the same time also reasons to believe it can hinder it. Either way the combo does more good than harm just take the creatine w/ some sugar or something. I always take my creatine a few after my ECA, and it certainly works at least as far as pumps go. So I know its doing something, don't know if I push out any addition reps though but thats just an individual creatine thing.