Quantum-chemical calculations of diazodicarbonyl compounds cycloadditions with C=S bonds of thioketones are in good agreement with the related experimental data. According to calculations, these reactions proceed via concerted cycloaddition producing thiocarbonyl ylides. For all diazodiketones and diazoketoesters, possessing at least one acyl group, the thermodynamically favorable process is 1,5-electrocyclization of intermediate C=S ylides into oxathiole, while for diazo compounds with two alkoxycarbonyl groups the dominating process is 1,3-electrocyclization to produce thiiranes, which at elevated temperatures can give rise to alkenes. The reversibility of 1,5-electrocyclization of C=S-ylide, generated from diazomalonate and thiocyclobutanedione, is dictated by a small activation energy of the first, cycloaddition step.