Quantum-chemical calculations of dimethyl-, trimethyl-, and methylethyladamantyl cations have been performed using the density functional theory method DFT B3LYP/6-31G* with full optimization of energy and computation of normal vibration frequencies and changes in these parameters during the course of their mutual isomeric transformations. The geometric parameters of the compounds, electronic characteristics, electron density distribution, the total energy, transformation energies, transformation entropies, activation energies, and normal vibration frequencies have been calculated. The calculation results confirm earlier experimental data on the occurrence of the direct 2,4-migration of the methyl group during the isomerization of methyladamantyl cations and the preference of this rearrangement to the well-known 1,2-methyl shift in alicyclic carbocations.