2-(Alkylthio)carbonyl-2-(arylethyl)propyl cobaloximes were photolyzed. When an aryl group is phenyl, a major process is a radical cyclization to give 2-(alkylthio)carbonyl-2-methyltetralin. On the other hand, β-thiolactone is a major product when an aryl group is mesityl and an alkyl group is t-butyl.