From the complete description of neat systems and the determination of phase diagrams, the influence of a nonfunctionalized initially soluble thermoplastic on epoxy-amine kinetics was discussed, considering the dilution effect induced by the additive. The initially soluble thermoplastic, a poly(ether imide), PEI, can act as a catalyst when the initial reactivity of the epoxy-amine system is very low. The conversion, x(cp), at which phase separation occurs depends on the initial miscibility of PEI in the mixture of monomers. Both structural transformations, gelation and vitrification, were found to be affected by the phase separation process because of the induced fractionation phenomena. The evolution of phase diagrams which cure was also considered and temperature-conversion-transformation (TxT) diagrams were presented.