The flash vacuum thermolysis of cycloalkenyl allyl sulfides, potential precursors of unsubstituted cycloalkenethiones, has been investigated by photoelectron spectroscopy. This technique allowed us to characterize in the gas phase very reactive species of this series, the conjugated and nonconjugated cyclopentene and cyclohexenethiones, and to compare the reactivity of these compounds. The assignment of the PE spectra is supported by electronic structure calculations, particularly by the HF method at the MP2 level and the density functional theory (DFT) using the B3LYP and BP86 functionals with the 6-311G(d, p) basis set. Copyright 2001 Academic Press.