Herein, we report the influence of an organophosphonite antioxidant, Irgafos P-EPQ on the thermal behavior of optimized poly(ether ether ketone)/poly(ether imide) (PEEK/PEI; 50/50) blend. The antioxidant addition was found to limit the rate of thermo-oxidation of the blend to a noticeable extent as revealed from TGA measurements and limiting oxygen index values. The DSC results indicate a decrease in T-g and melting enthalpy of the blend with the antioxidant addition. The DSC results were verified from MDSC and WAXS measurements. It is suggested that addition of 1 wt.% Irgafos P-EPQ to PEEK/PEI blend results to good thermo-oxidative stability. The effective activation energy (E-alpha) of thermo-oxidation process for 1 wt.% Irgafos P-EPQ incorporated blend decreased steadily than its pure counterpart, in consonance with its improved thermo-oxidative stability. (C) 2012 Elsevier B.V. All rights reserved.