"Preheating with hot, inert gas appears to be a promising method to prepare modified-in-situ oilshale retorts for ignition. Continuing earlier work (1*), we have used computer simulation to determine the effects of four factors on preheating time, on retorting rate and peak temperatures reached after changeover, and on the 'bank"" of char generated in the topmost layers of the rubble. The factors are: grade of shale, void fraction of rubble, particle size, and composition of retort gas (percent of air in air/steam mixture). Maximum gas temperature rose sharply with grade and percent air, weakly with particle size; char bank rose with grade and volume fraction of solids, as expected; retorting rate rose most strongly with percent air and void fraction, while preheating time decreased with increasing void fraction and grade. Simple empirical equations provide excellent fits to the results of the complex model calculations over the practical range of the factors."