The occurrence of Norrish type-I α-cleavage has been established in β-dithiolactones, involving the n-π* singlet state (diradical and carbene reactive intermediates). Concerted ring expansion to a thiacarbene from the excited state is suggested to be responsible for the formation, at least in part, of one of the cyclic thioacetals. Although this α-cleavage process is similar to that of corresponding