Molecular orbital calculations up to the MP4/6-311++G(2df,2p)//MP2/6-31G(d,p) + ZPE level have been undertaken on the isomerization/dissociation processes of oxonium cations [CH3OH2](+), [(CH3)(2)OH](+), and [(CH3)(3)O](+). In addition to the classical oxonium cation structure, two ion-neutral complexes were identified : [H2CHO(R)... CH3](+) and [H2C=O(R)... CH4](+) (R = H, CH3). The former is only a weakly stabilized structure while the latter (stabilized by 10-15 kJ/mol) is a reaction intermediate during the direct hydride ion abstraction [CH3]+ + CH(3)OR --> CH4 + [CH(2)OR](+). The barrier height associated with the 1,2-elimination reaction [H2CHO(R)-CH3](+) --> [H2C=O(R)](+) + CH4 is calculated to be 267 and 295 kJ/mol for R = H and CH3, respectively. This energy barrier is mainly due to the heterolytic C-O bond dissociation energy, and as a result, the corresponding transition structure looks like a [CH3](+)... CH(3)OR complex. Calculation has also been used to predict a heat of formation of the trimethyloxonium cation : Delta H-f degrees(300K)[O(CH3)(3)](+) = 536 +/- 15 kJ/mol.