The dewetting behavior of thin polystyrene (PS) films on top of an amorphous polyamide (PA) layer is investigated at two different annealing temperatures (119 and 195 degreesC). While PS is liquid at both temperatures, the PA layer remains solid at 119 degreesC and becomes highly viscous at 195 degreesC. At a constant thickness of the PA. layer, the thickness and the molecular weight of the PS top layer were varied. With optical microscopy and optical phase interference microscopy ex situ the hole growth was measured during the dewetting of the PS layer. A statistical analysis of the data shows evidence for a change in the dewetting mechanism from a spinodal process into a nucleation and growth process as a function of film thickness. For very thin films the effect of a geometrical constraint is discussed. The retardation of the observed dewetting process was achieved by adding a styrene maleic anhydride random copolymer (SMA). The influence of the amount of SMA added and its molecular weight are shown for each given temperature. SMA with a low weight percentage content of maleic anhydride shows an increased retardation as compared to a higher weight percentage irrespective of the amount of SMA added.