"Inert-gas preheating appears to be a good way to start the retorting of oil-shale rubbles in situ. This wok was undertaken to determine a property that is important for that process: the temperature at which spent-shale char spontaneously ignites in air and diluted air. Spent Colorado oil shale obtained from inert-gas retorting of 118-R./Mg (28-gal/ton) oil shale, was preheated with nitrogen in a small reactor, then exposed to oxygen/nitrogen mixtures for periods of about 20 minutes. Temperatures recorded from embedded thermocouples and the concentrations of carbon oxides in the effluent gas were followed as indicators of the ignition process. The factors explored in a 2'3-factorial design were temperature of preheating, 300 and 350 C; average particle size, 0.45 and 1.80 cm; and concentration of oxygen in influent gas after changeover, 10.45 and 20.9 vol. %. Two runs were made with the gas flow downward; in the other ten it was upward. Rapidly rising temperatures, coupled with substantial concentrations of CO and CO2 in the effluent gas indicated that ignition did occur in all the runs. In three runs, temperatures close to 6000C were recorded and there was evidence that even higher temperatures may have been reached in these and other runs. The rate of temperature rise increased with increasing oxygen concentration and particle size. CO and CO2 both rose with increasii1g influent oxygen concentration, but the ratio of CO to CO2 dropped sharply, possibly because of catalytic conversion of the one to the other. Responses were the same were the gas flow was upward or downward . The results obtained in this work, coupled with those of earlier simulation studies , indicate that inert-gas preheating should be a safe, economical and reliable method for initiating retorting in in situ oil shale rubbles."