Epoxies with high cross-linking densities are brittle and hence have a low fracture toughness. However, different methods are known to increase their fracture toughness, such as by incorporating a second phase (i.e., rubber, inorganic nanoparticles, or thermoplastics) into the epoxy matrix, referred to as bulk resin modification. The potential alternative to conventional bulk resin modification is the introduction of thermoplastic multilayers into the epoxy system. In the case of thermoplastic film, the interphase region is created by diffusion and dissolution, followed by reaction-induced phase separation. However, the influence of various factors, such as (i) the curing history, (ii) PEI film thickness, and (iii) PEI/epoxy volume fraction on the microstructure evolution in the interphase region of the system, is not well understood. Therefore, the main aim of the presented research work is to understand the interphase formation as a function of the abovementioned parameters in a pure PEI/epoxy system, to later attain the desired interphase morphology and its particle size for improved material toughness in fiber-reinforced PEI/epoxy composite. The resulting structure in the case of PEI/epoxy system displays three distinct areas: i) an epoxy phase with small PEI inclusion, ii) a phase inverted gradient interphase with epoxy inclusions in a PEI matrix (also referred to as interphase), and iii) remaining pure PEI which did not interact with the epoxy. The concentration gradient in the interphase leads to a gradient morphology resulting from the phase separation. The interphase thickness increases as a function of both 1st dwell curing temperature and PEI film thickness. In the case of fiber-reinforced PEI/epoxy composite, the PEI veil was partially dissolved at a lower temperature while fully dissolved at a higher temperature. The phase inverted particles were present between the fibers. The fracture toughness of the PEI/epoxy system was enhanced due to the synergetic toughening mechanism. This synergy comes from the inhomogeneity effect, which resulted in a crack arrest. In the case of fiber-reinforced PEI/epoxy composite, it was observed that the fracture toughness was higher at a lower temperature. The morphology gradient at the interphase and phase inverted particles between fibers provided toughening due to the several mechanisms (i.e., shear yielding, crack deflection, etc.)