A commercial linear low density polyethylene (1-hexene comonomer) was reacted with dicumyl peroxide at appropriate levels to initiate chain extension without crosslinking. The peroxide modification was carried out both in the presence and absence of a common hindered phenolic antioxidant. Thermal treatment of the resin was also completed with and without antioxidant present to evaluate the contribution of classical thermooxidative degradation to the peroxide modification process. In terms of pure thermal degradation, the phenolic antioxidant was found to be effective in prevention of reduction of molecular weights at the high end of the molecular weight distribution, as reflected in changes in M(z). At the low end of the distribution, the opposite effect is seen, where the removal of the antioxidant actually allows an increase in the value of M(n), through chain coupling reactions. Processing of the resin with addition of the organic peroxide significantly increases the molecular weights of both antioxidant containing and antioxidant free resins. Increasing the reaction temperature increases the rate of degradative chain scission and nonenlarging disproportionation reactions. At low temperatures, the presence of the antioxidant acts negatively with respect to chain enlargement reactions, and an overall reduction is seen in the efficiency of the peroxide in its role of increasing molecular weights. As the reaction temperature is increased, chain scission reactions become more predominant, and the antioxidant free resin shows very substantial decreases in molecular weight. Retention of the antioxidant at higher reaction temperatures is beneficial to maintaining high molecular weights, both with and without peroxide present. As in the peroxide free case, branching is essentially unchanged by the absence of antioxidant in peroxide modification.