A high-pressure (0-1000 psig) thermobalance experiment has been developed in order to obtain thermogravimetric (TG) and derivative thermogravimetry data (DTG) pertinent to the in-situ combustion process for enhanced oil recovery. Studies on the pressured thermo-oxidative behaviour of 20/80 (W/W) oil/sand and coked sand samples are reported. In the case of the oil/sand sample under a 100 psig non-oxidising atmosphere, distillation/evaporation occurred below 320 degrees C at which point thermal cracking commenced. Above 520 degrees C only carbon-rich coke material remained. Under an oxidising air atmosphere at 100 psig, the TG curves were more complex. Below 360 degrees C, low-temperature oxidation (LTO) competes with distillation/evaporation; hence the rate of weight loss is significantly decreased whilst fuel deposition occurs. Between 360 degrees C and 475 degrees C, the observed weight loss is believed to be due to a combination of hydrocarbon combustion and visbreaking or coking. Above 475 degrees C, combustion of the remaining organic matter (high-temperature oxidation HTO) occurs. Experiments conducted using coked sand samples showed that, in air at 100 psig, the sample weight increased up to 250 degrees C, This is attributed to the formation of oxygenated species. Above 250 degrees C, the sample rapidly lost weight due to combustion reactions.