A series of tests was made on Colorado oil shale to determine the effect on oil yield by preheating the shale with hot gases prior to retorting. The following factors affecting the oil yield of the shale were studied: (1) particle sizes of samples ranging from -48 460 mesh to -2 44 mesh, (2) heating temperatures from 147 degrees to 300 degrees C, (3) heating periods from 15 minutes to 36 hours, and (4) heating gases containing from 0 to 21 percent oxygen.