The behavior and the structural properties of organic radical species (ORs) in their native hydrocarbon environment as a function of temperature was investigated by electron paramagnetic resonance spectroscopy (EPR). Five petroleum fractions derived from the same initial crude oil were studied in the temperature range of 293-673K. Results from continuous wave (CW) and pulsed EPR spectroscopy show petroleum fractions behave differently under thermal activation. The inverse of the normalized area versus the temperature gave us different slopes changes in accordance with the generation and the recombination of ORs. The EPR experimental parameters (lineshape, linewidth) highlighted that during the thermal treatment, no conformational change occurred but impact the organic matter maturity with the breaking of sulfur chains.