Sandia National Laboratories and Geokinetics, Inc. have recently completed processing of a 6000 ton 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 district operating conditions, including air at high and low flow rates and air plus 30% combusted recycle gas. A number of techniques were used to evaluate the effectiveness of these various processing modes. Te extensive suite of thermal instrumentation, for example, allowed continuous mentoring of the steam and retorting fronts and estimation of oil coking losses and total shale retorted. Actual sweep efficiency, based upon these data, was 79% Sweep efficiency estimates from steam front data were nearly identical, both in total shale contacted and spatial distribution of the swept zone. The thermal data also provided a direct means of assessing the validity of a number of retort diagnostic techniques based on fluid product analyses. Off gas material balance calculation estimates of sweep efficiency, for example, were 77% 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 rubble shale beds with low void volume is practical using horizontal burn techniques and that existing retort diagnostics are capable of providing a detailed analysis of the process.