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