The alkaline hydrolysis of a thio-beta-lactam in the gas phase was examined in the light of RHF and DFT ab initio calculations. The solvent effect was considered via IPCM computations. The tetrahedral intermediate for the thio-beta-lactam studied is unstable, so the compound evolves directly to the corresponding thio-azethidin-2-one open ring with cleavage of the C-S bond. The end-products obtained bear a carbamate group, which suggests that the thio-beta-lactam might be an effective inhibitor for beta-lactamases.