The effects of injection gas composition of the performance and economics of a Modified In Situ oil shale retort have been examined. The injection of air could produce oil at a slightly lower price than steam-air injection. Retort systems using steam oxygen or carbon dioxide oxygen are economically unattractive. A system using water and air could be economical if water addition can be controlled. Approximately one-quarter of the energy is produced as a retort off-gas that has a very low heating value.