We have studied the dynamics of dewetting at the interface between polystyrene (PS) and poly(methyl methacrylate) (PMMA) as a function of the molecular weights of the two materials. Optical microscopy and atomic force microscopy have been used to follow the dewetting process and to image both the surface and interfacial structure in the vicinity of the growing holes. We determined the scaling behavior of the dewetting velocity as a function of the molecular weight of the dewetting species (PS). Different regimes are found depending on the PMMA molecular weight. For large PMMA molecular weights the dewetting velocity scales inversely with the PS viscosity, while for low PMMA molecular weights, the dewetting velocity is almost independent of the PS viscosity. The experimental data are in quantitative agreement with recent theoretical predictions.