The photodissociation dynamics of the acetone S-2(n,3s) Rydberg state has been studied by using femtosecond pump-probe multiphoton ionization mass spectrometry. Acetone was excited at 195 nm and the temporal evolutions of the initial state and intermediates were monitored. We considered two mechanisms: (1) the conventionally accepted mechanism, in which the primary dissociation occurs on the first triplet-state surface; and (2) the newly proposed mechanism by the Zewail's group, in which the primary dissociation takes place on the first singlet-state surface. The conventional mechanism was found to be inadequate to describe the observed dynamics. On the other hand, our results and analyses supported the validity of the new mechanism. (C) 2003 Elsevier B.V. All rights reserved.