The evolution of oil during pyrolysis of blocks and powders of oil shale was investigated using heating rates from 2 to 180/sup 0/C/h. Blocks (in the form of right circular cylinders) ranged in size from 3.2 to 17.2 cm in diameter and from 58 to 204 1/t (14 to 49 gal/ton) in grade. The results show that coking reactions are the major source of intraparticle oil degradation. Furthermore, the degree of coking depends strongly on the heating rate, but is nearly independent of particle size and grade for the conditions used. The experimental results from the 17.2 cm blocks are analyzed using a simple mathematical model based on kinetic results from powder retorting experiments and thermal data reported in the literature. The model calculations agree closely with the experimental data. Finally, the practical implications of this work for field retorting are discussed.