The influence of the molecular mass of two incompatible polymers on their miscibility in the solid state has been studied using Scanning Electron Microscopy and Optical Microscopy. Two pairs of incompatible polymers, polystyrene/poly(methyl methacrylate) and polystyrene/poly(oxyethylene) were cast from a nonselective solvent. When the two polymers are of comparable and relatively high molecular mass (M-w similar to 60,000) we obtain films in which the domains of the two phases are reduced to 7 mu m, without the use of any compatibilizer. On the contrary, when the two polymers present very different molecular masses, a lamellar structure is obtained due to the high repulsion between them. This repulsion has already been observed in solution (Refs. 4 and 5). In the case of polymers with comparable but very low molecular masses the repulsion between them, in solution, is also high, leading to phase separation in the films obtained after removal of the solvent.