The structure of perhydroacenaphthene (I) and its conversion products 1,3-dimethyladamantane (II), trans-1,4-dimethyladamantane (III), cis-1,4-dimethyladamantane (IV), 1,2-dimethyladamantane (V), 1-ethyladamantane (VI), and 2-ethyladamantane (VII) of the general formula C12H20 has been studied using the Becke-Lee-Yang-Parr (B3LYP) hybrid method for the energy functional of the electron density with the 6-31G* basis set. Geometric and electronic characteristics of the compounds and their total energy, trans-formation energies, transformation entropies, and normal vibration frequencies have been calculated. The equilibrium constants determined for the isomerization of perhydroacenaphthene into products II-VII are consistent with experimental data.