The in situ forward combustion process was evaluated using both air and steam-oxygen injection. Bench scale tube reactor experiments were conducted on Asphalt Ridge (Utah) tar sand to study the effects of the nitrogen or steam component in the injected gas on the oil yield, product oil quality, and process efficiency. Total-injected-gas and oxygen fluxes of 40 and 8 scf/h-ft/sup 2/, respectively, were similar during both combustion experiments. Regardless of the injected gas composition, the oxygen consumption rapidly approached 100% and the combustion process became self-sustaining at ignition temperatures of about 650 to 750/sup 0/F (343 to 399/sup 0/C). This was because the partial pressure of oxygen was identical for both experiments. A carbonaceous fuel was burned after ignition and the combustion temperatures increased to 1200 to 1800/sup 0/F (649 to 982/sup 0/C). Steam injection reduced coke formation, and the steam-char reaction further reduced the coke available for combustion in the steam-oxygen experiment. These mechanisms resulted in reduced fuel availability, less oxygen consumption, increased combustion zone velocity, and a slightly greater oil yield for the steam-oxygen test. The oil yield for the steam-oxygen combustion test was 67 wt % of the initial bitumen, which was slightly greater than that from the air-only experiment. Other beneficial effects of steam versus nitrogen during the combustion process were improved heat transfer, reduced differential pressures, and greater heating value of the product gas for the steam-oxygen test. 11 refs., 2 figs., 7 tabs.