The effect of water vapor on the yield and composition of products formed at thermal treatment of Bulgarian oil shale kerogen at atmospheric pressure has been studied. It was found that water steam reacts with oil shale organic matter significantly affecting the yield and composition of liquid and gaseous products. As for the yield of liquid products, it is about 20% higher than the yield obtained at Fischer Assay. The yield of semicoke is about 10% less. Liquid products formed in the presence of water vapor are richer in aromatic and aliphatic hydrocarbons among which long-chain paraffins with up to 33 carbon atoms predominate.