The photolysis of C2H5SH liquid at 2537 A has been shown to give H2 and C2H5SSC2H5 at equal rates. The photolysis of ethanethiol - methyl disulfide liquid mixtures leads, via a chain reaction involving propagation by attack of thiyl radicals on the disulfide S--S bond, to the formation with high quantum efficiency of CH3SH, C2H5SSC2H5 and, as an intermediate that is consumed after long exposures, CH3SSC2H5. The net result of the sequence of exchange processes is the essentially irreversible conversion of the methyl disulfide into methanethiol. The same overall reaction occurs thermally at room temperature, but the rate is appreciably less than that of the photochemical process. The quantum yields of formation of the unsymmetrical disulfides arising from the photochemically initiated exchange reaction in equimolar mixtures of CH3SSCH3 + n-C3H7SSC3H7, CH3-SSCH3 + C2H5SSC2H5, and C2H5SSC2H5 + n-C3H7SSC3H7 have been determined. In all three types of system examined in this investigation all thiyl radicals can be accounted for stoichiometrically on the basis of exchange and combination reactions alone, indicating negligible disproportionation of these species in condensed phase. (Modified author abstract).