Dodecanethiol-derivatized nanoparticles of Ag, Ag/Au, and Au with an average size of 5.5 nm have been prepared via a two-phase synthetic route in water/toluene and characterized with various methods. In particular, their photochemical dynamics and mechanisms have been studied with laser flash photolysis. The surface plasmon absorption peak varies linearly from 532 to 429 turn as the Ag fraction of Ag/Au alloy clusters, in which Ag atoms are more abundant at the outer part, increases. Excited silver nanoparticles dispersed in cyclohexane undergo dethiolation in 8 ns and then fragmentation in 40 ns. Thereafter, the fragments gradually aggregate owing to the presence of less soluble bare metal sides.