Composites of sulfonated polystyrene (PS-SSA) (0-8 mol % sulfonation) mixed with submicronic styrene-4 vinylpyridine (PS-4VP) (31 mol % 4VP content) microspheres were investigated by differential scanning calorimetry, Fourier transform infrared spectroscopy, and dynamic mechanical spectroscopy at 1 Hz in the glass transition region. The resulting proton transfer reaction from the SSA to the 4VP units was confirmed by IR spectroscopy, and led to a significant increase in the post-T-g Young’s modulus as well as a lengthening of the rubbery plateau. Surprisingly, the addition of sulfonated polystyrene microspheres to styrene-vinyl pyridine copolymers had no such effects, possibly because of steric factors.