Sandia National Laboratories and Geokinetics, Inc. have recently completed processing of a 6000 tonne horizontal in situ oil shale retort near Vernal, Utah. The heavily-instrumented, explosively-fractured oil shale bed, containing 12% void, was combustion retorted using a number of distinct operating cinditins, including air at ;high and low flow rates and air plus 30% combused recycle gas. A number of techniques were used to evaluate the effectiveness of these various processing modes. The extensive suite of thermal instrumentation, for example, allowed continuous monitoring of the steram and retorting fronts and estimation of oil coking losses (from heating rate data) and total shale retorted. Actual sweep efficiency, based upon these data, was 79%. Sweep efficiency estimates from steam front data werenearly identical, both in total whale cintacte and spatial distribution of the swept zone. The thermal data also provided a direct means of assessing the validity of a numbver of retort diagnostioc techniques based on fluid porduct (offgas and oil) analyses. Offgas material balance calculation estimates of sweep efficiency, for example, were 776 , while retorting efficiency was 58% of Fischer assay, with a 27% oil loss to combustion and a 15% loss to coking. Oil loss estimates based on oil analyses were similar. The experiment demonstrated that true in situ retorting of thin seam rubbled shale beds with a low void volume is practical using horizontal burn techniques and that existing retort diagnostics are capable of providing a detailed analysis of the process.