This article deals with the thermal aging of a polyethylene-co-methyl acrylate elastomer. Six different activation energies have been determined corresponding to the following different mechanisms : surface oxidation, whole bulk oxidation in the case of heterogeneous oxidation, consumption of antioxidant, creep, and relaxation (compression set). Relaxation mechanism has been found to have the lowest activation energy( 45 kJ/mol). Dominant chemical modification has been found to have the highest activation energy (119 kJ/mol). The case of heterogeneous oxidation is considered in details : a variation of the activation energy is obtained. The activation energy varies from a constant upper value related to homogeneous oxidation, to a lower value related to the so-called "total heterogeneous oxidation.