Results concerning gamma-irradiation of polymer blends such as HDPE/ground tire rubber (GTR) and PP/HDPE are reported in this article with a special emphasis on the order of processing steps. Irradiation dose varied in the range 0-100 kGy. The two first polymers (HDPE and rubber) are preferentially crosslinked under gamma irradiation while PP undergoes chain scission. Mechanical tests and differential scanning calorimetry (DSC) analysis show that the efficiency of the reactive compatibilization by gamma irradiation depends greatly on the chronology of gamma-irradiation and injection-molding steps. Electron spin resonance (ESR) results reveal that numerous radicals remain trapped in the materials after gamma-irradiation even after a long time. Then the effect of irradiation on material properties is different if polymers are melted after irradiation or not. Crosslinking and chain scission are not affected in an equivalent way by the order of processing steps. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1710-1717, 2010