Polystyrene-based nanocomposite films containing up to 20 wt% laponite clay have been prepared by emulsion polymerization. Significant increases in the storage and tensile moduli were observed in both the glassy and rubbery state on laponite addition. However, whereas in the glassy state these increases were correlated with the extent of exfoliation of the laponite, in the rubbery state they were more dependent on the overall laponite content. These results are discussed in terms of the observed morphologies and micromechanical models for the reinforcing effect of rigid nano-sized filler particles.