Poly(butylene terephthalate) (PET) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations in concentrations ranging from about 3 wt% to 8 wt%. The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4’-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA) and 4,4’-diaminodiphenylsulfone (DDS). Using conversion-temperature transformation diagrams developed in part 1, thermal cycles were selected to generate different morphologies. In the case of PBT-DGEBA-DDS systems, phase separation in the course of reaction led to a random dispersion of spherical particles (sizes in the range of 1 mu m), rich in PET. Small and wide angle X-ray scattering, carried out in situ, during cure, revealed that the dispersion of spherical particles was produced by a nucleation-growth mechanism and that crystallization took place after phase separation. A completely different morphology, characterized by a distribution elf large and irregular semicrystalline particles, was produced by crystallization before reaction. However, both types of morphologies introduced a small increase in the critical stress intensity factor. The main toughening mechanism was crack bridging produced by highly drawn thermoplastic particles. On the other hand, PBT-DGEBA-MCDEA formulations were cured at temperatures high enough to avoid crystallization of PET during reaction. In this case, the PET remaining dissolved in the matrix did not introduce any toughening effect.