Thin films of polystyrene (PS)/polymethylmethacrylate (PNMA) blends have been spin cast on mica from chloroform solutions. When the concentration of PMMA in the casting solution is less than that of PS a pitted morphology is formed. The average sizes of the pits are shown to increase with both the total concentration of the casting solution and the relative concentration of PMMA. The change in pit size is explained in terms of incomplete dewetting of a PMMA solution from an underlying PS solution. For a given ratio of PMMA/PS the average pit diameters appear to increase linearly with the square of the film thickness, the gradient of which is dependent on the film composition.