Thermal oxidation of raw natural rubber from Hevea brasiliensis was analyzed by differential scanning calorimetry (DSC) in dynamic mode. The results obtained revealed a linear relation between the PRI and onset temperature obtained by DSC (r(2) = 0.91), for PRI values <70. On the other hand, when the natural rubber PRI was over 70, DSC was unable to assess that criterion accurately. This was probably because the two techniques were not equivalent regarding the phenomenon measured. Oxidation primarily results in chain scissions during PRI measurement. In DSC, the transition seen during thermal oxidation was associated with crosslinking. In fact, thermo-oxidation is in the initial phase when PRI is measured, but in the terminal phase at the onset of transition in DSC.