The five-membered heterocyclic gamma-butyrothiolactone was isolated in a low-temperature, inert Ar matrix, and the UV-visible (200 <= lambda <= 800 nm)-induced photochemistry was studied. On the basis of the IR spectra, the formation of ethylketene (CH3CH2CH=C=O) was identified as the main channel of photodecomposition. The valence electronic structure was investigated by He(I) photoelectron spectroscopy assisted by quantum chemical calculations at the outer valence Green's function/6-311++G(d,p) level of theory. The first three bands at 9.54, 9.69, and 11.89 eV are assigned to the n(S), n(O), and pi(C=O) orbitals, respectively, denoting the importance of the -SC(O)- group in the outermost electronic properties. The conventional ring strain energy was determined to be 3.8 kcal mol(-1) at the G2MP2 level of calculation within the hyperhomodesmotic model.