A series of polyurethane (PU)/SiO2 composites were prepared via in situ polymerization through a prepolymer process. The effect of SiO2 content on mechanical and solvent-resistant properties of PU/SiO2 composites was investigated. It was found that with increasing SiO2 content, the tensile strength and toughness, hardness, and modulus of PU composites increased. PU/SiO2 composites exhibited only one loss peak corresponding to the glass transition temperature of the soft-segment of PU, which shifted insignificantly with increasing SiO2 content. The equilibrium swelling ratio and swelling rate constant in cyclohexanone and xylene were reduced by increasing SiO2 content, indicating the enhancement of the solvent resistance of the PU elastomer. Morphology observation of PU/SiO2 composites showed that the acicular SiO2 dispersed uniformly in PU matrix, and there was no obvious aggregation even at 9 wt % loading of SiO2. The reinforcing and toughening effects depended largely on the dispersion of SiO2 in PU matrix and the interfacial layer formed between the two phases. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 2732-2738, 2013