We have used optical microscopy and scanning force microscopy to study the dewetting of polystyrene from poly(methyl methacrylate) on silicon substrates as a function of film thickness. We have performed measurements for bilayers with a lower layer more viscous than the upper layer, as well as for the opposite situation. For a solidlike (highly viscous) lower layer, the dewetting speed is constant and independent of the thickness of the polystyrene film. However, for a liquidlike lower layer, the radius of the dewetted holes grows as t(2/3), where t is the annealing time, and depends on the thickness of both layers. The absolute values of the dewetting speed are in reasonable agreement with theoretical predictions.