Oil shale solids are pyrolyzed or retorted by contact with a non-oxidizing recycle gas in a first retorting zone to produce an effluent containing pyrolysis products and pyrolyzed oil shale solids. Liquid and gaseous product hydrocarbons are recovered from the effluent of the first retorting zone. The pyrolyzed oil shale solids are contacted with an oxygen-containing gas in a combustion zone under conditions such that at least a portion of the organic material in the pyrolyzed solids is burned to produce hot, decarbonized solids and a hot flue gas. At least a portion of the gases in the effluent from the first retorting zone is contacted in the substantial absence of molecular oxygen with a portion of the hot, decarbonized solids produced in the combustion zone in the presence of added hydrocarbon-containing fines, preferably oil shale fines, in a second retorting zone such that the gases are heated and the fines are pyrolyzed to produce pyrolyzed fines and pyrolysis products containing gases and vapors. The gases and vapors in the effluent from the second retorting zone are then recycled to the first retorting zone where they serve as the hot, non-oxidizing recycle gas.