© 1997 SPE All rights reserved.In-situ combustion technology is considered to be an efficient one not only for heavy oil reserves but also for depleted light and medium oil bearing reservoirs. Unfortunately, the lack of better understanding of the process variables in terms of the conversion of oil during combustion and reservoir characteristics, as well as the costs, limits the more effective application of this technology. In this study, SARA fractions of two (medium and heavy) Turkish crude oils were separated by column chromatographic techniques and combustion experiments were carried out on whole oils and fractions by thermogravimetric analyser (TG/DTG) and differential scanning calorimeter (DSC) under air atmosphere at 10 °C/min. heating rate. TG and DSC data were analysed for the determination of weight loss parameters, and for heat data of individual fractions (which have to be known for in-situ combustion technology utilisation) and to determine the temperature domains for individual reaction for each fraction. Investigation of SARA fraction for combustion enables us to show quantitatively the temperature intervals where evaporation, oxidation and combustion effects operate for each fraction, data shows that each fraction has specific and sometimes overlapping temperature domains for these reactions. Experiments show that the most similar fractions in terms of their behaviour are aromatics and resins. Asphaltenes and saturates are two different extremes. For instance, asphaltenes go into combustion directly with almost no low temperature oxidation, whereas saturates are the earliest (at low temperature) oxidised compound. It has a low temperature combustion region which, in a way, triggers combustion of whole oil. If we think that during pyrolysis and combustion, large amount of saturates is produced, which means, it is role ahead of combustion front will affect the stability of process. By using the data and findings of this study one can take advantage of studying the kinetics of SARA fractions instead of complex whole oil for modelling of the overall process accurately to predict combustion.