The domain morphology of a polystyrene/poly(methyl methacrylate)/poly(2-vinylpyridine) (PS/PMMA/ PVP) polymer blend was studied after spin-coating a film from a common solvent. The strongly incompatible polymer mixture phase-separates during spin-coating. Atomic force microscopy, combined with selective dissolution of the different polymer phases was used to obtain information on the polymer distribution in the film. By changing the relative composition of the mixture, a great variety of different morphologies was observed. The common feature of all observed morphologies is the compatibilizing function of poly(methyl methacrylate), which intercalates between the more incompatible polystyrene and polyvinylpyridine domains to prevent the formation of high-energy interfaces.