The spiro-fused oxadiazoline, 3,4-diaza-2,2-dimethyl-1,6,10-trioxaspiro[4.5]dec-3-ene, when thermolysed in a sealed tube in benzene-d6 at 110 °C, afforded acetone and an apparent oxirane intermediate (2,2-dimethyl-1,4,8-trioxaspiro[2.5]octane) that could not be isolated. Attempts to isolate the oxirane gave a dimer (8,8,11,11-tetramethyl-1,5,7,10,12,16-hexaoxadispiro[126.96.36.199]hexadecane) as the major product. The oxirane is thermally stable at 110 °C but it is very sensitive to water, as indicated by its gradual disappearance after the tube was opened. The dimer of the oxirane is believed to form from a cation arising from the ring-opening of the oxirane when it reacts with moisture. This cation then reacts with the oxirane itself to regenerate water, which is effectively a catalyst for conversion of the oxirane to the dimer.Key words: carbene, dioxacarbene, dialkoxyoxirane, oxadiazoline, 2,5-dihydrooxadiazole.