The influence of filler loading, size and surface treatment on the dynamic mechanical behaviour and thermal properties of styrene-methacrylic acid copolymer/glass bead composites is analysed. A theoretical approach to quantifying molecular mobility is proposed to explain the decrease in molecular motion due to strong matrix-filler linkages. The higher the filler content and the smaller the particle size, the greater the number of anchored bonds. This effect is enhanced for beads coated by silane or an elastomeric agent. In the latter case, the reduction in molecular motion is less evident owing to an increase in the mobile sequence length, which induces greater mobility.