"The effects of concurrent steam and air injection during oil shale retorting were experimentally investigated. The oil yield increases as the steam concentration of inlet gas increases, and reaches the maximum when the steam concentration is in the range of 30 to 40%. Results indicate that the steam increases the combustion rate of residual carbon in the spent shale, the thermal decomposition rate of inorganic carbonates, and the reaction rate of residual carbon with the carbon dioxide inside the spent shale when the steam concentration of inlet gas is higher than oxygen concentration. It was also found that the steam reduces the oil loss by thermal cracking and coking as well as the oil loss during the revaporization at the raw shale surface. The steam increases the advancing rates of the peak temperature and retorting zone as well as hydrogen production per ton of oil shale by reacting with the carbon monoxide rather than with the residual carbon inside the spent shale."