To model the formation processes of C-10-C-20 petroleum isoprenanes, thermolysis of regular and irregular C-20-C-40 isoprenanes (phytane, crocetane, squalane, and lycopane) and the suggested precursors of regular petroleum isoprenanes (phytol and isophytol) has been conducted. It has been shown that the thermolysis of these compounds results in the formation of regular, irregular, and pseudoregular isoprenanes; the trends in their distribution have been revealed; and the retention indices have been determined. It has been assumed that the pristane/phytane ratio, which is used in petroleum geochemistry, should be treated with caution.