Tertiary a-carbomethoxy-a,a-dimethyl-methyl cations a have been generated by electron impact induced fragmentation from the appropriately a-substituted methyl isobutyrates 1-4. The destabilized carbenium ions a can be distinguished from their more stable isomers protonated methyl methacrylate c and protonated methyl crotonate d by MIKE and CA spectra. The loss of I' and Br' from the molecular ions of 1 and 2, respectively, predominantly gives rise to the destabilized ions a, whereas loss of C1' from [3]" results in a mixture of ions a and c. The loss of CH,' from [4]" favours skeletal rearrangement leading to ions d. The characteristic reactions of the destabilized ions a are the loss of CO and elimination of methanol. The loss of CO is associated by a very large KER and non-statistical kinetic energy release (T,, = 920 meV). Specific deuterium labelling experiments indicate that the a-carbomethoxya,a-dimethyl-methyl cations a rearrange via a 1,QH shift into the carhonyl protonated methyl methacrylate c and eventually into the alkyl-0 protonated methyl methacrylate before the loss of methanol. The hydrogen rearrangements exhibit a deuterium isotope effect indicating substantial energy barriers between the [C,H,O,]+ isomers. Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.