The morphology of a polypropylene/polyethylene (PP/PE) blend and a maleic anhydride modified PP and PE (PPg/PEg) blend was studied. The initial morphology, at the extruder die, after the melt blending into a twin-screw extruder, was first characterized. Then, the evolution of this initial morphology was followed after a injection molding operation, and during annealing in an oven at 200degreesC. The influence of the compatibilization of the blend by coupling reactions through covalent (with 1,12-diaminododecane) and ionic reactions (with zinc acetate and sodium hydrogenocarbonate) was also investigated. At the extruder die, the viscosity ratio proved to be a determinant factor governing the dispersed phase diameter of the droplets, and as a second factor, the addition of small amounts of coupling-agents together with (PPg/PEg) to (PP/PE) resulted in a decrease of the diameter of the droplets. The injection molding of these initial blends resulted in important coalescence and in an elongation of the dispersed phase. This was observed for the non compatibilized and also for some of the compatibilized blends. The ionic coupling showed a good stabilization of the morphology. Finally, the morphology of the non compatibilized blends was found to be instable when the material was annealed at 200degreesC. The average size of the dispersed phase increased. The coupling reactions delayed the occurrence of the coalescence about 5 min and limited its effects. The extent of the coarsening depended strongly on the composition of the blend and on the nature of the coupling. Still, the ionic agents appeared more effective. (C) 2004 Wiley Periodicals, Inc.