The thermodynamic equilibrium concentrations of petroleum C-15 polymethylated bicyclo[4.4.0]decanes (sesquiterpanes) at 500 and 600 K have been experimentally obtained for the first time via the aluminosilicate-mediated equilibrium isomerization of the saturated 200-300 degrees C fraction of crude oils of different genotypes. It has been found that sesquiterpane isomers with an angular carbon atom and methyl and ethyl substituents in the molecule are the least thermodynamically stable in the equilibrium mixture and their relative concentration increases with increasing temperature from 500 to 600 K; the most stable isomer is 2,3,3,7,7-pentamethylbicyclo[4.4.0] decane.