During 1981, Occidental Oil Shale, Inc., conducted two approximately 12 m x 12 m x 16 m vertical modified in situ retorting experiments designated MR-3 and MR-4. These retorts were run to develop ignition technology for their commercial size retorts 7 and 8. Sandia participated in the experiments by providing both thermal instrumentation and a computer based data acquisition and anlaysis system. An analysis approach utilizing both thermal data and material balance calculations based on offgas composition was applied to both retorts. Results indicate that the analysis tools used can provide a detailed assessment of retort performance. Thermal instrumentation provided detail about the structure of the retort front. Material balance calculations permitted estimates of retorting rates and oil loss mechanisms such as coking and combustion. The sweep efficiencies were estimated to be 76% for MR-3 and 48% for MR-4. Oil loss to combustion in the two retorts were 35% and 29% of Fischer Assay, respectively. Material balance results also permitted detailed energy balances to be performed on the retorts.