Liquid and gaseous products are recovered from an in situ oil shale retort containing a fragmented permeable mass of formation particles by establishing a combustion zone in the fragmented permeable mass of particles. The combustion zone is advanced through the fragmented mass of particles by introducing a retort inlet mixture comprising oxygen into the retort on the trailing side of the combustion zone. The retort inlet mixture is maintained at a sufficiently high temperature of at least 1150.degree. F. so that the temperature of at least a portion of the fragmented mass on the trailing side of the combustion zone is maintained at a temperature of at least 1150.degree. F. for increasing the yield of hydrocarbon products obtained from the retort.