Ab initio molecular orbital calculations have been employed to study the unimolecular rearrangement of 3,3-disubstituted 1-methyl-5-oxo-4,5-dihydro-3H-1,2,4-triazolium cations to protonated 1,5-disubstituted 2-methyl-1,2-dihydro-1,2,4-triazol-3-ones. The 1,2-migration of three different substituents (methyl, ethyl, and isopropyl) has been investigated on the basis of a three-center transition-state model. Geometries of stationary points on the potential energy hypersurface were optimized at the HF/6-31G* level of theory. Second-order Moeller-Plesset perturbation theory was applied with the 6-31G* basis set in order to correct for correlation effects. On the basis of HF/6-31G* and MP2/6-31G*//HF/6-31G* activation energies, migratory aptitudes are predicted in the order isopropyl >> ethyl >> methyl, which is in excellent agreement with experimental data reported for the rearrangement of 1-aryl-substituted 1,2,4-triazolium analogues. Partial charges obtained from the natural population analysis indicate strong electron deficiency at the nitrogen atom in the triazolium cations being the target for migration. The transition states show partial carbocation character of the migrating group with respect to charge distribution and geometry.