Oxygen-containing compounds that are known to be precursors of petroleum hydrocarbons have been subjected to catalysis on an aluminosilicate catalyst. It has been found for the first time that along with C-11-C-12 monoalkylated adamantanes, which are typically present in crude oils, the homologous series of 1- and 2-n-alkyladamantanes up to and including n-heptyladamantanes are generated via the catalysis (monoalkyl-substituted adamantanes with a substituent heavier than the ethyl radical have never been found in any crude oil). It has been shown that, with the increasing degree of conversion, C-13-C-17 adamantanes disappear and only 1- and 2-methyl- and ethyladamantanes remain in the catalyzates. The absence of high-molecular alkyladamantanes (C-13 and higher) in crude oils is supposedly attributed to relatively high maturity of crude oil.