The effects of interfacial adhesion strength on the mechanical properties of composites of polypropylene and glass particles were investigated. The 3.5 mum average diameter glass particles were surface-treated using two silanes with different functional groups. The functional groups were hydrocarbons, expected to promote adhesion between filler and matrix, and fluorocarbons, expected to reduce the strength of adhesion. Mixtures of the functional groups were also used to treat the surface of the glass to obtain better control of adhesion strength and thus the mechanical properties of the composites. A model study using glass slides and polypropylene films was conducted to confirm the feasibility of treatment. Adhesion strength between glass and polypropylene increased with increasing coverage of the hydrocarbon silanes. The surface-modified particles were incorporated into the polypropylene matrix via melt processing. While surface functionalization of the particles can influence the dispersion of the particles, no significant effect was observed in this study. Tensile tests and toughness tests were performed on injection-molded samples. The tensile strength of the reinforced polypropylene increased with increasing adhesion strength. Impact toughness increased with weaker adhesion but the dependence became less pronounced as deformation rate was increased. (C) 2004 Elsevier Ltd. All rights reserved.