Various mechanisms for the alkaline hydrolysis of an aza-beta -lactam in the gas phase were studied by ab initio calculations at the RHF/6-31+-G*//RHF/6-31+G* MP2/6-31+G*//MP2/6-31+G*, and B3LYP/6-31+G*// B3LYP/6-31+G* levels, Solvent effects were considered via IPCM (isodensity polarizable continuum model) calculations at the IPCM/6-31+G*//RHF/6-31+G* level. The alkaline hydrolysis of beta -lactams begins with a nucleophilic attack of the hydroxyl ion on the carbonyl of the beta -lactam ring. The tetrahedral intermediate thus formed undergoes cleavage of the C-7-N-4 bond to give the reaction end products. In addition to the typical cleavage reaction, the beta -lactam studied can undergo opening at the C-7-N-6 bond (Scheme 1). Both processes have a similar activation energy that varies slightly depending on the particular computation method used. The most stable end products are those formed via the typical mechanism. In any case, both mechanisms yield products possessing a carbamate group, which suggests that the starting aza-beta -lactam might be an effective inhibitor for beta -lactamases.