Dynamic mechanical spectra of polystyrene (PS) or styrene-co-methacrylic acid copolymer (SAMA) filled by various amounts of raw or coated glass beads have been recorded in the glass transition (T-g) region of the polymers used as the matrix. The relative contribution of the two key features governing the magnitude of the main mechanical relaxation, i.e., (i) the mechanical coupling between phases, and (ii) the physico-chemical interactions at the polymer/filler interface, is separated by an original mechanical modeling, which gives evidence only for the reinforcement effect of the polymer matrix. Thus, by comparing experiment and theory, it was shown that the decrease in the tan delta maximum displayed by PS reinforced by high volume fractions of raw glass beads mainly results from an improvement of the reinforcement effect, because of the presence of clusters of particles. In contrast, for SAMA copolymer used as the matrix, it was found, in addition, a decrease in the molecular mobility of chains due to interactions at the polymer/coated glass bead interface. Differential scanning calorimetry and scanning electron microscopy analysis agree with this conclusion.