The photochemistries of the molecules C1C(O)SC1 and C1C(O)SCH3 (in both their normal and perdeuterated forms) isolated in solid Ar or N-2 matrixes at 15 K have been investigated. On the basis of evidence of the IR spectra of the matrixes, the products of irradiation with broad-band UV-visible light (200 less than or equal to lambda less than or equal to 800 nm) were identified, thereby revealing quite different photochemical behaviors for the two molecules. C1C(O)SC1 is subject to multichannel changes that include interconversion of the syn and anti rotamers, photodecomposition to give CO and SCI2, formation of the hitherto unknown radical C1C(O)S-., and subsequent decomposition of this radical to give either the C1CO(.) radical or the OCS molecule. By contrast, C1C(O)SCH3 decomposes in two steps, the first consisting of fragmentation to CO and C1SCH(3), and the second entailing detachment of a hydrogen atom from the methyl group of C1SCH(3) With the formation of the molecular complex H2C=S...HC1. The IR spectra of both the C1C(O)S-. radical and H2C=S...HC1 have been interpreted with reference to the spectra predicted by ab initio and density functional theory methods.