The monitoring of the combustion front in in-situ combustion (ISC) processes is important for the precise control of the air injection to make sure that the combustion process occurred in the expected direction. Our previous study showed that the free-radical signal obtained by Electron Paramagnetic Spectroscopy (EPR) measurements might be applied to determine the direction of combustion front. Therefore, in this study, the feasibility of using EPR as a new and fast technique for monitoring combustion front was further investigated. For this purpose, the oxidation process of different types of crude oils was studied in the low-temperature range from 25 to 180 °C. Free radicals concentration was measured using EPR technique for each sample before and after the isothermal oxidation. The results showed that there is some dependence between the free radicals concentration and temperature. For all the tested crude oils, the concentration of free radicals produced in oxidation process increased with temperature. In a real air injection process, the combustion process inevitably leads to an increase of reservoir temperature. The zone in the moving direction of the combustion front will have a higher temperature than other zones. The increased free-radical concentration can be an indication of the increase of reservoir temperature. Therefore, it is possible to estimate combustion front by analyzing free radicals signals using EPR technique. However, there is no universal relationship being found in this study for different types of crude oils because the sensitivity of EPR technique to oxidation processes strongly depends on the composition of crude oil. That is to say, for each crude oil, we need to find its own dependence between reservoir temperature and free radicals signal for the estimation of the combustion front. Also, it was found that a higher content of asphaltenes resulted in a more significant change of the free radicals concentration with temperature as the asphaltenes can stabilize and increase the lifetime of free radicals due to the presence of π-systems. This implies that the monitoring of the combustion front using EPR technique will be easier and more precise for the oil with a high content of asphaltenes.