Thin polymer blend films of deuterated polystyrene (dPS) and poly(p-methylstyrene) (PpMS) with several blend compositions between 95:5 and 5:95 were prepared on top of glass substrates. The samples were examined right after preparation and after annealing. Their surface morphology was investigated with scanning force microscopy (SFM). A statistical analysis of the SFM data yielded the rms roughness as well as the most prominent in-plane length parallel to the sample surface. Information about the density profile perpendicular to the sample surface is gained from neutron reflectivity measurements in the region of total external reflection. Right after preparation dPS segregates to the air interface, while during annealing at a temperature above the glass transition temperature of both polymers, PpMS builds up a top layer toward the interfaces. Depending on blend composition and annealing time, different surface topographies are evolving. The examined film thicknesses are below a critical thickness, ensuring that composition fluctuations directed normal to the surface are suppressed. However, the small film thickness constrains kinetics largely to two dimensions. The surface morphology is controlled by an interplay between phase separation and dewetting.