The reaction course of the oxidative decomplexation of Fischer carbene complexes with dioxiranes was examined. The portionwise addition of 2.2 equiv of dimethyldioxirane (DMD) to Fischer carbene complex 1 afforded ethyl phenylpropiolate in 90% yield. When the reaction was carried out using a CO(2)-free DMD solution in a N(2) atmosphere ester 2 was formed in 40% yield, whereas in the presence of an O(2) atmosphere the yield increased to 70%. This same assay performed in the presence of (18)O(2) atmosphere afforded the ester 2 partially labeled at the C=O moiety (approximately 50%, GC-MS) with (18)O. On the other hand, treatment of Fischer carbene complex 1 with a [(18)O(2)]dimesityldioxirane solution led to the formation of (18)O-labeled CO(2) (trapped as BaCO(3) and detected by IRMS). From these results it can be suggested that the oxidative decomplexation of Fischer carbene complexes by dioxiranes involves an initial attack of the dioxirane to the metal coordination sphere. In this step a CO ligand is oxidized to CO(2) thus leaving an unstable chromium tetracarbonyl intermediate which would react with O(2) to give the final ester product and chromium(III) oxide.