Summary This paper presents the results of 10 enriched-air in-situ combustion-tube tests performed on core from the Athabasca oil sands deposit. The tests show that at high pressures, the use of oxygen-enriched air results in increased low-temperature reactions between the oxygen and the oil, resulting in an increased fuel load and decreased burn stability. Although water injection may enhance the performance of oxygen combustion, it may also lead to increased oxygen storage in the swept zone. Introduction In-situ combustion, or fireflooding, is a thermal recovery technique that offers a high theoretical efficiency and uses relatively simple surface equipment. Although the process has been extensively studied in the laboratory and the field, operational problems associated with field projects, combined with the success of steam-based processes in a wide variety of reservoirs, have retarded the large-scale application of this technology. Enriched-air injection is a modification of standard in-situ combustion technology with the potential to overcome a number of the problems that have hindered widespread field application of the process. In their excellent review of oxygen fireflooding literature, Garon et al. concluded that oxygen offers many process and operational advantages over normal air in-situ combustion. Fairfield and White discussed the advantages of using oxygen in the heavy-oil reservoirs of Saskatchewan, where field operations were hampered by severe sanding and gas locking of the production wells and by the formation of tight, difficult-to-treat emulsions. They concluded that removal of the large nitrogen volumes from the reservoir by injection of oxygen rather than normal air would not only reduce the sanding and gas-locking problems, but the greater allowable rates of oxygen injection would result in accelerated recovery schedules. It is now generally accepted that in-situ combustion should be a follow-up process to steam stimulation if it is to be applied in cold, viscous reservoirs, such as those of northern Alberta. BP Resources Canada Ltd., in conjunction with the Alberta Oil Sands Technology and Research Authority (AOSTRA), has successfully piloted oxygen injection following steam stimulation of the Cold Lake reservoir. The application of in-situ combustion as an alternative to steam flooding has considerable merit because of its significantly higher thermal and displacement efficiencies. This paper discusses enriched-air (95% oxygen) in-situ combustion tests conducted for AOSTRA on the Athabasca Oil Sands reservoir by the U. of Calgary In Situ Combustion Laboratory. The original objective of these tests was to evaluate the burning characteristics of enriched air at pressures up to 5520 kPa [800 psi]. The definite pressure effects noted during the initial test program, however, prompted an extension of the pressure range to 10 300 kPa [1,500 psi].