Commercially available neoalkoxy titanate (Lica 12) and organosilane (3-aminopropyltriethoxysilane) coupling agents were used to treat talc and calcium carbonate (CaCO3) to compare their effects with those of untreated fillers upon their incorporation into polypropylene (PP). Commercial stearic acid treated CaCO3 was also used to widen the scope of the study. Single-filler PP composites (containing either talc or CaCO3) and hybrid-filler composites (containing a mixture of talc and CaCO3) were compounded on a twin-screw extruder and subsequently injection-molded into dumbbells. The silane and titanate treatments dramatically increased the elongation at break for both the single-filler and hybrid-filler composites, whereas stearic acid did not. There was also a moderate improvement in the impact strength of the composites, particularly those treated with Lica 12. The hybrid composites, through the synergistic coalescence of positive characteristics from talc and CaCO3, had exceptionally good impact properties, more so with the aid of the coupling agents. For example, the impact strength value of a Lica 12 treated hybrid composite was the greatest for all the composites studied, overshadowing the superiority of the CaCO3-filled PP composites, which predominantly had the highest impact properties. Further investigations of the thermal and morphological properties were also conducted to facilitate the determination of the coupling mechanisms and their interesting effects on the hybrid composites. (c) 2005 Wiley Periodicals, Inc.