Three types of mineral fillers, gypsum, wollastonite, and talc, were investigated for their ability to modify the mechanical properties of commingled recycled-plastic composites containing 0.07-0.26 v/v of filler. Mechanical test results showed that the talc reinforced composites were significantly better in mechanical properties when compared with the gypsum and wollastonite composites. Scanning electron microscopy (SEM) showed that gypsum formed large agglomerates in the matrix. Interfacial adhesion between filler and matrix was evaluated using simple empirical models. To enhance the adhesion, talc, and wollastonite were pretreated with silane coupling agents, 3-methacryloxypropyltrimethoxy silane (gamma-MPS) and 3-aminopropyltriethoxy silane (gamma-APS). This did not result in any significant improvement to the material properties. The gamma-APS treatment, however, increased the tensile properties of the composites by similar to 5% when compared with the gamma-MPS treatment. The SEM investigations showed that the gamma-APS treatment provided better adhesion of filler particles and hence voids were less likely to form in the matrix when compared with the gamma-MPS composites. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3470-3481, 2009