In this work the morphology of spin-coated polymer films is explained based on the competitive interactions between polymer, solvent and substrate. The films were formed on silicon wafers, a polar surface. Toluene and tetrahydrofuran (THF) were chosen for the dissolution of polystyrene (PS), poly(vinyl chloride) (PVC) and poly(vinyl butyral) (PVB). PS spin-coated films from solutions prepared in toluene were homogeneous and flat, while PS and PVC films obtained from solutions prepared in THF were very rough and presented segregation on the surface, as revealed by the atomic force microscopy. However, PVB films prepared from THF are continuous and homogeneous. When the interaction energy between substrate and solvent overcomes that between substrate and polymer, the films become rough and segregate. On the contrary, when the interaction energy between substrate and polymer is stronger than that between substrate and solvent, or when both interaction energies are weak, the films obtained are homogeneous and flat. These homogeneous and flat films were also characterized by ellipsometry and contact angle measurements.