Relatively inert diazodiketones react with aliphatic thiones in a way similar to the reaction with aromatic thioketones to produce the corresponding spirocyclic oxathioles. The most probable reaction mechanism involves cycloaddition of the dipolar diazo group to the C=S bond of thione, followed by 1,5-electrocyclization of intermediate C=S ylides. Dimethyl diazomalonate reacts in this way at 19–23°C, whereas at elevated temperatures 1,3-electrocycli-zation of C=S ylide and subsequent desulfurization of thiirane lead to tetrasubstituted olefins as final products.