The N-benzyl- and N-alkyl-substituted 1,2-thiazetidin-3-one 1,1-dioxides 1b - d reacted readily with NH3 and primary amines via ring opening. The reaction with NH3 proceeded at -78° to room temperature yielding ring-opened adducts via nucleophilic attack of NH3 at the sulfonyl group, whereas the reactions with amines at room temperature yielded products via attack at the carbonyl group. The N-unsubstituted analogue 1a, when reacted with benzylamine in refluxing EtOH, also gave a product of ring opening via nucleophilic attack at the carbonyl group of 1a. The transamidation-like reactions of the 2-(aminoalkyl)-1,2-thiazetidin-3-one 1,1-dioxides 19a - d proceeded via six-, seven-, and eight-membered intermediates, giving the ring-enlarged eight-, nine-, and ten-membered products 21 - 24 (Schemes 8 and 9), respectively, in 42 - 87% yields. The products resulted from the nucleophilic attack of the amino group of the side chain at the carbonyl C-atom. The structure of the eight-membered product 24 with an asymmetrically situated methyl substituent was established by X-ray crystallography.