Results for thermal cracking of shale oil vapor over shale are presented. Cracking temperatures of 504 to 610/sup 0/C and residence times of 2 to 11 seconds were studied. A first-order Arrhenius rate expression and associated stoichiometry were given. Changes in oil properties caused by thermal cracking are also discussed. Cracking increases nitrogen content and decreased the H/C ratio and pour point of the recovered oil. Spectroscopic evidence related the decrease in H/C ratio to an increase in aromatic and alkene content of the oil. Gas-phase oil cracking was contrasted to liquid-phase oil coking as a loss mechanism in oil shale retorting. Results for oils prepared in the laboratory were also compared with oil prepared in the TOSCO-II semi-works and in modified and true in-situ combustion retorts. It was demonstrated that the naphthalene/2-methylnaphthalene ratio is a good indictor of cracking temperature in an oil shale retort.