Bitumen and its heptane-asphaltene and heptane-petrolene fractions extracted from P.R. Spring (Utah) tar sand have been investigated using EPR at 9.2 GHz. In each case a single, structureless EPR transition is observed at g≈2 due to organic free radicals. From the temperature dependence of the free radical spectrum at 10–300 K, it is argued that, in addition to monoradicals, exchange coupled spins exist in all three fractions which result in a singlet ground and a triplet excited state. The origin of the exchange interaction, which is isotropic, is discussed in terms of the presence of similar heteroatoms in alternate aromatic sheets. The separation, Δ=127 ± 1 cm −1 , between singlet and excited triplet states for all three fractions indicates a similar core structure which is consistent with Pfeiffer and Saal's model for bitumen. Oxygen and/or air causes a reversible enhancement of the free radical signal for asphaltene which is explained in terms of enhanced exchange coupling. An irreversible change in the asphaltene structure is observed to take place on subjecting it to a thermal cycle of 292-500-292 K, indicating a complex reorientation of the structure during cooling.