Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH 3 O-C-OCH 2 CF 3 •+ , 1 •+ , are cleanly generated by the dissociative electron ionization (EI) of 2-methoxy-5,5-dimethyl-2-(2,2,2-trifluoroethoxy)-Δ 3 -1,3,4-oxadiazoline I. Neutralization–reionization (NR) mass spectrometry of the neutral carbene 1, generated by one-electron reduction of 1 •+ , shows no recovery ion signal and thus 1 is not a viable species within the μs time scale of the experiment. Very low vapour pressure (VLVP) pyrolysis – mass spectrometry of I in conjunction with (multiple) collision experiments shows that 1 completely isomerizes, via a 1,2-trifluoroethyl shift, into methyl 3,3,3-trifluoropropionate, CF 3 CH 2 C(=O)OCH 3 , 1a. This technique was also used to study the related dialkoxycarbenes C 2 H 5 O-C-OCH 2 CF 3 , 2, CH 3 O-C-OC 2 H 5 , 3, and CH 3 O-C-OCH(CH 3 ) 2 , 4, generated from the corresponding 2,2-dialkoxy-5,5-dimethyl-Δ 3 -1,3,4-oxadiazolines. The pyrolytically generated carbene 2 behaves analogously to 1 and completely isomerizes to ethyl 3,3,3-trifluoropropionate, 2a. The neutral carbenes 3 and 4 undergo only a partial isomerization via 1,2-alkyl shifts in which the ethyl and isopropyl groups show a slightly greater migratory aptitude, respectively, than the methyl group. The differences in migratory aptitude are explained in terms of a transition state model similar to that of a 1,2-H shift in carbenes, with development of negative charge in the migrating group. The greater migratory aptitude of CF 3 CH 2 , as compared to CH 3 and CH 3 CH 2 , is attributed to the stabilization of negative charge in the transition state by strongly electron-withdrawing β-fluorines whereas the differences in migratory aptitude between the alkyl groups in 3 and 4 are largely due to the greater polarizability of isopropyl and ethyl groups, as compared to the methyl group. Key words: dialkoxycarbenes, pyrolysis, tandem mass spectrometry.