Oil shale from the illite-rich Garden Gulch Member of the Green River Formation in Colorados Piceance Basin was pyrolyzed in a self-purging 12 kg batch autoclave. The shale was held within the autoclave in a container open only at the top, and a nitrogen sweep around that container swept the produced gas and oil vapors out of the headspace at a controlled pressure determined by a back-pressure regulator. Oil and water weights and gas composition were measured as a function of time, and oil composition was evaluated by a variety of techniques. Increasing back pressure retarded the evolution of generated oil. This volatilization phenomenon can be fit simply by adjusting the activation energy of the pyrolysis kinetics by 0.67 cal/mol/kPa. Oil yields decreased and gas yields increased with increasing back pressure. Oil quality increases as yield decreased. API gravity was typically above 40, nitrogen content was only 0.50.6 wt %, and As, V, and Ni were below detection limits of 0.1 ppm. The quality improvement with lower yield is attributed to longer liquid residence time for coking, which results in less heavy oil being volatilized. Results are consistent with the original work using this approach by Burnham and Singleton (Burnham, A. K.; Singleton, M. F.High Pressure Pyrolysis of Green River Oil Shale. In Geochemistry and Chemistry of Oil Shales; Miknis, F. P., McKay, J. F., Eds.; ACS Symposium Series 230; American Chemical Society: Washington, DC, 1983.), who used Mahogany zone shale from the Anvil Points mine near Rifle, CO. These results demonstrate the common finding in fossil fuel processes that product quality and quantity trade off with each other.