Direct dissociation of hydrogen bonds in photoexcited adenine-water clusters was observed in real time by femtosecond pump-probe transient ionization time-of-flight mass spectrometry. When hydrated clusters of adenine were excited by femtosecond laser pulse at 262 nm, they lost all of the solvent molecules in ca. 200 fs, regardless of the number of the water molecules attached. It is suggested that the adenine-water hydrogen bond is dissociated on the repulsive part of the potential energy surface for the npi* state of adenine. This study presents a microscopic model for the dynamics of in vivo DNA bases after UV absorption. (C) 2002 Elsevier Science B.V. All rights reserved.