A laboratory evaluation of oil shale retorting using sulfur trioxide injection has shown that sufficient heat is generated to retort oil shale. But the direct reaction of sulfur trioxide and oil shale results in large oil yield losses. A screening evaluation of the economic feasibility for sulfur trioxide retorting indicated that the large oil yield losses increase shale oil production costs relative to conventional retorting processes. Sulfur trioxide may be an economical alternative to air for oxidation of retorted shale in retorting processes using solid heat carriers. In this case, the shale is heated largely by the exothermic reaction of sulfur trioxide with the mineral carbonates in the shale. The hot retorted shale can be mixed with raw shale to isolate the oil and gas production from the sulfur trioxide reactions. A screening evaluation of this alternative use for sulfur trioxide indicates that the by-product credits for high Btu off-gas and CO/sub 2/ may offset the additional production costs for the sulfur trioxide. 11 refs., 7 figs., 12 tabs.