Amino units were grafted onto the surface of small particle size alumina by reaction with 3-aminopropyltriethoxysilane. Atactic polystyrene (PS) was sulfonated (1-14 mol % sulfonation) and mixed with both modified and unmodified alumina at filler loadings varying from 30 to 80 wt %. The resulting composites were characterized by differential scanning calorimetry, Fourier transform infra-red spectroscopy, and dynamic mechanical spectroscopy in the glass transition region at a frequency of 1 Hz. Whereas mixtures of unsulfonated PS with either filler showed essentially no change in T(g) with filler content, sulfonated PS saw its T(g) increase as a function of filler loading, at a rate which was greater following modification of the alumina. At a fixed filler loading of 30 wt %, the composite rubbery plateau modulus was found to increase with copolymer sulfonic acid content, while the loss tangent maximum corresponding to the glass transition broadened and decreased. These observations were interpreted as a manifestation of the decrease in polymer mobility brought upon by the formation of noncovalent crosslinks resulting from the proton transfer from the sulfonic acid units on the polymer to hydroxyl and/or amino units at the surface of the filler.