The phase separation and dewetting of thin films of blends of deuterated polystyrene (dPS) and poly(p-methylstyrene) (PpMS) were investigated during annealing. The surface morphology, obtained from atomic force microscopy and phase measurement interference microscopy, the density profile, determined by X-ray reflectivity in the region of total external reflection, and the surface composition obtained from static secondary ion mass spectroscopy, are reported. This system is only weakly incompatible. The interaction of the components with substrate and air during phase separation leads to a bilayer formation with a broad polymer-polymer interface. PpMS segregates to the air interface. The bilayer structure is unstable and defines the starting point for the dewetting of PpMS on top of the dPS layer. In the final dewetting state a homogeneous layer of dPS on top of the substrate is covered with an ultrathin layer of PpMS as well as with quite thick mesoscopic drops of PpMS.