True in situ combustion retorting of oil shale in fractures has been simulated in six experiments by igniting the common surfaces between two instrumented blocks of oil shale. Experimental details are given for all experiments although only one of the six experiments became self-sustaining. Three ignition systems were used involving heating the injected gas, heaters between the blocks and heaters in the blocks. The data obtained allow estimation of the heating rates of the oil shale at various places, the rate of combustion zone movement, the amount of shale retorted, and the width of the combustion front. The data show that the "fracture" between the blocks can be closed down by condensed or coked oil, shale swelling, shale spallation, and shale fusion. Results indicate that longer blocks would be desirable in further experimentation.