Planar interfaces have been prepared by assembling high-jr, thermoplastics such as poly(phenylene ether) (PPE) or poly(ether imide) (PEI) and epoxy-amine networks at different conversions of the epoxy groups, X-e, followed by a complete curing. Direct evidence of the structure of the interfacial regions was made by means of electron microscopies, SEM and TEM, and atomic force microscopy, after a complete curing of the epoxy-amine. The morphologies of the interfaces are explained from the competition between the dissolution process of a thermoplastic by nonreactive solvent and the thermodynamics and the kinetics of reactive epoxy-amine/thermoplastic system. If the phase diagram indicates that the epoxy-amine/thermoplastic system is nonmiscible, the interface remains very thin (< 20 nm). in the opposite, a gap range of miscibility controlled by the polymerization rate allows a diffusion of the comonomers into the thermoplastic layer and its further dissolution. As a consequence, 100-400 m thick interphases can be generated. A gradient morphology results from the gradient composition at phase separation. The gradient composition or concentration profile is shown to depend on the type of dissolution process that is mainly determined by the competition between the thermodynamic osmotic pressure and the swelling resistance of the thermoplastic.