The alkylation products of IIIa and b with chloro- acetone exhibited different characteristics. From IIIa, the thioacetone existed in its normal form, with spectral data as indicated. From IIIb, the same reaction gave a product that existed in the thioacetone form VII in solution (DMSO) only. When the ir was recorded in solid state (Nu- jol), no carbonyl absorption was observed. Instead a hydroxyl absorption was observed, indicating the ex- istence of a hemiketal.