The method of assaying oil shale by the modified Fischer retort has been used for several thousand determinations at the Laramie, Wyoming and Rifle, Colorado stations of the Bureau of Mines. In the light of this experience a more detailed description of the procedure is given which includes precautions that should be emphasized for proper appraisal of oil shale. Although these precautions are normally practiced by the careful analyst, experience has shown the advisability of calling attention to them as a uniform guide for the assay of oil shale.