The crystal structures of dimethyl 2,2,4,4-tetramethyl-3-oxocyclobutane-1-spiro-5 -3 ,4 -diphenyl-(1,3)-thiazolidine-2 ,2 -dicarboxylate, C 26 H 29 NO 5 S for V, and dimethyl 3,4-diphenyl-5-isopropylidene-(1,3)-thiazolidine-2,2-dicarboxylate, C 22 H 23 NO 4 S for VI, have been solved. The 1,3-thiazolidine ring of compound V has got twisted conformation, while in compound VI this ring adopts envelope. In both structures short inter-and intramolecular contacts were found, which can be recognized as hydrogen bonds.