This study was performed with commercially available phenyl trimethoxysilane (PTMS) and neoalkoxytitanate [i.e., neopentyl(diallyl)oxytri(dioctyl)phosphato titanate (LICA 12)] as coupling agents. PTMS and LICA 12 were used to treat talc and kaolin to compare their effects with untreated fillers upon incorporation into polypropylene (PP). Single-filler PP composites (containing either talc or kaolin) and hybrid-filler composites (containing a mix of both talc and kaolin) were compounded in a twin-screw extruder and subsequently injection-molded into dumbbells. The incorporation of PTMS and LICA 12 slightly decreased the tensile and flexural properties in terms of modulus and strength but increased the elongation at break for both single-filler and hybrid-filler composites. There was also a significant 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 kaolin with the aid from chemical treatment provided an economically advantageous material with mechanical properties comparable to those of the single-filler-filled PP composites. Further investigations on flow and morphological properties were also done to correlate the mechanical properties of the single- and hybrid-filler-filled PP composites. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2770-2779, 2008