We present calculations for the mechanism and the barrier heights of tautomerization of adenine. We find various pathways for the 9(H) <-> 7(H) and 9(H) <-> 3(H) tautomerization. One mechanism for the 9(H) -> 7(H) tautomerization involves an sp(3)- or carbene-type intermediate, whereas the other proceeds via imine intermediates. Tautomerization from the 9(H) tautomer to 7(H) or 3(H) is predicted to occur with a very large activation barrier (60-70 kcal/mol), indicating that the processes may not occur readily in the gas phase. Interactions with the water molecule(s) are found to lower the barrier tremendously. We suggest that dramatic lowering of the 9(H) -> 3(H) and 9(H) -> 7(H) barriers by microsolvating water molecules may facilitate the formation and observation of the 7(H) and 3(H) tautomers in the solution phase.