A series of composites of commercial polyurethane (PU) and hybrid silica have been prepared by the sol-gel process through the in situ synthesis of hybrid silica in a solution of dichloromethane. Mechanical properties, small angle x-ray scattering (SAXS) and differential scanning calorimetry measurements were performed to evaluate the effects that hybrid silica has on the properties of the resulting PU composites. A series of 13 different organosilane differing in polarity and alkyl chain length has been employed. Compared with pristine PU, composites bearing hybrid silicas were more likely to exhibit reductions in the yield stress and increase in the elongation at break. Samples also showed a negligible variation in the glass transition temperature and a reduction in the ΔCp. The enthalpy of the crystallization process showed a trend towards reduction. Based on the SAXS measurements, the composites showed a small reduction in the interdomain spacing, an increase in the degree of phase separation and a tendency towards reduction in the size of the interphase thickness between domains. These results can be better explained as being caused by the volume of the organic group from the organosilane rather than by the chemical interaction of the organic group with the PU organic moieties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41157.