The evolution upon curing of interdiffusion as well as phase separation is characterized at the interphase between a high T-g poly(ether sulfone) thermoplastic (TP) and a high density thermosetting network (TS), both components being initially miscible. We show that the late stages of the diffusion process overlap with the early stages of phase separation, which is contrary to the expectations from the current state of the art. The penetration depth of the TS into the TP is found to increase with decreasing gelation time within the investigated operating window. This is due to the stronger thermal activation of diffusion over reaction at the early stages of curing. A chemometric model is developed based on Raman spectroscopy data in order to further analyze these results. The predictions of the model are compared with the profiles generated by image analysis carried on microtomed sections of fully cured interphases. The concentration profiles of partially cured interphases are predicted as well and used to correlate the degree of curing of the TS with the front velocity for each profile. (C) 2016 Published by Elsevier Ltd.