The rate of CO and CO/sub 2/ evolution from Colorado oil shale heated at a constant rate under an autogenous atmosphere is reported. Three samples with grades of 84, 150 and 251 l/Mg were investigated. The gas evolution rates from powdered samples in an autogenous atmosphere are nearly identical with those from solid cores. The rate of dolomite decomposition depends strongly on the grade of the oil shale and the presence of a sweep gas. Results are tentatively interpreted in terms of the accessibility of the dolomite crystals to steam produced during retorting.