In connection with the unimolecular decompositions of ionized isopropyl methyl ether, namely, methane elimination and methyl radical loss, ab initio molecular orbital calculations at the UMP2, QCISD, and QCISD(T) levels of theory with the 6-31G(d) and 6-31G(d,p) basis sets have been used to investigate the relevant parts of the C4H10O.+ ground-state potential energy surface. The calculations demonstrate that at internal energies below the threshold for loss of a methyl radical, bond dissociation to an ion-neutral complex, [CH3O+=CHCH3 CH3.], and methane elimination therefrom both occur. Sets of reactant and transition state frequencies calculated at the UMP2/ 6-31G(d) level, as well as energetics based on QCISD(T)/6-31G(d,p)//UMP2/6-31G(d) + ZPVE calculations, were employed in RRKM calculations. By invoking quantum-mechanical barrier tunneling in the methane elimination channel, we were able to reproduce several experimental observations : (a) the methane elimination reaction dominates at internal energies near the threshold for dissociation; (b) the onsets for the methane and methyl losses differ by only a few tenths of a kilocalorie per mole; and (c) there is a very strong primary isotope effect of about 60 on the CH2D2 and CHD3 losses from CH3OCH(CHD2)(2)(.+).