We have found that, while 2-oxetanone undergoes thermal decarboxylation at 350 degrees C to give CO2 and ethene, 2-azetidinone proceeds in two different modes at 550 degrees C, giving isocyanic acid, ethene, ketene, hydrogen cyanide and carbon monoxide among the pyrolysis products. B3LYP/6-31G** calculations were performed to investigate the two alternative cycloreversion pathways for the two compounds. Theoretical thermodynamic data predicted that decarboxylation of 2-oxetanone has among the lowest activation barrier (159 kJ mol(-1)) and free energy (Delta G = - 183 kJ mol(-1)). While the cycloreversion to ethene can be classified as a concerted asynchronous process, natural bond orbital (NBO) analysis suggested that the cycloreversion to ketene is more asynchronous for 2-azetidinone than for 2-oxetanone.