The energetics and decomposition mechanisms of (CH3)(2)(CH5)CO-and (i-Pr)(C2H5)(2)CO- have been studied at the G2++, G3, and G3(MP2) levels of theory. It is found that the energies required for methane elimination of (CH3)(2)(C2H5)CO- are less than that for ethane elimination, in agreement with experiment that only the former reaction is observed. On the other hand, the energies required for ethane and propane eliminations of (i-Pr)(C2H5)(2)CO- are within 2 kJ mol(-1) of each other. So both eliminations are likely, as observed experimentally. Mechanistically, it is found that alkane dissociation of the tertiary alkoxide anions follows a pathway similar to that of methane elimination for primary and secondary alkoxides. The dissociation starts with detachment of a carbanion. followed by proton abstraction to form an alkane molecule, with an ion-molecular complex being formed as an intermediate.