Some possible oxygen-containing precursors of petroleum hydrocarbons were subjected to mild thermolysis. It has been first shown that n-alkylcyclohexanes and steranes are formed along with the expected hydrocarbons n-alkanes and isoprenanes, which result from the loss of a functional group, and there is good correlation between the molecular-mass distributions of n-alkanes and n-alkylcyclohexanes. It has been found experimentally that sterane fragments of oxygen-containing compounds are thermodynamically unstable, and this is confirmed by the formation of biological C(27)-C(29) 5 alpha,14 alpha,17 alpha,20S- and 20R-, and 5 beta,14 alpha,17 alpha,20R-(coprostane) steranes as a result of thermolysis.