The deuterium isotope effect in the photodissociation of acetone S-2 state was studied using femtosecond pump-probe ionization spectroscopy. The transients obtained for both isotopomers can be well described by the same mechanism in which the primary dissociation occurs on the S, surface. Substantial isotope effects were observed in every stage of the reaction. Our results indicted that upon full deuteration the initial-state decay from S2 to S I Slows down by a factor of three, the subsequent adiabatic dissociation on the S I surface slows down by a factor of four, and the secondary dissociation slows down by a factor of similar to 1.6. (c) 2005 Elsevier B.V. All rights reserved.