The four-membered heterocyclic beta-propiothiolactone compound 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 methylketene (CH3CH=C=O) was identified as the main channel of photodecomposition. The formation of ethene and thiirane, with the concomitant elimination of OCS and CO, respectively, was also observed as minor decomposition channels. The valence electronic structure was investigated by HeI photoelectron spectroscopy assisted by quantum chemical calculations at the OVGF/6-311++G(d,p) level of theory. The first three bands at 9.73, 9.87, and 12.06 eV are ascribed 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. Additionally, the structure of a single crystal,grown in situ, was determined by X-ray diffraction analysis at low temperature. The crystalline solid [monoclinic system, P21/c, a = 8.1062(1) angstrom, b = 10.3069(2) angstrom, c = 10.2734(1) angstrom, beta = 107.628(1)degrees, and Z = 8] consists of planar molecules arranged in layers. The skeletal parameters, especially the valence angles [(sic)C2-C1-S = 94.55(7)degrees, (sic)O=C-C = 134.20(11)degrees, (sic)C-S-C = 77.27(5)degrees], differ from those typically found in acyclic thioester compounds, suggesting the presence of strong strain effects. The conventional ring strain energy was determined to be 16.4 kcal/mol at the G2MP2 level of calculation within the hyperhomodesmotic model.