Porosity and permeability reduction during oil shale retorting is well documented in the literature. The reduction in permeability has been observed in low void vertical modified in situ (VMIS) field experiments with adverse effects. Mechanical forces caused by the weight of the overlying shale can equal 100 psi near the bottom of commercial retorts. Postburn investigation of the Rio Blanco retorts at the C-a lease tract in Colorado shows evidence of compaction occurring. Western Research Institute is conducting research to study compaction of large oil shale blocks. The data presently available suggest that most oil shale compaction occurs during retorting between 600 and 800/sup 0/F and that significant amounts of compaction may occur in regions of VMIS retorts where 30 gallon per ton or richer oil shale is retorted. Recent oil shale compaction research has studied the effects of pressure, mineral decomposition, and a wide particle size distribution on oil shale compaction. Results from these tests indicate that compaction increases with an increase in overburden pressure. However, the weight of the rubble column is sufficient to cause compaction of the richer oil shales throughout a VMIS retort. Mineral decomposition and particle size distribution have little effect on the overall compaction of the spent shale. 14 refs., 5 figs., 3 tabs.