Poly(butylene terephthalate) (PET) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations. 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) or 4,4’-diaminodiphenylsulfone (DDS). PET was more miscible in DGEBA-MCDEA than in DGEBA-DDS formulations, as revealed by the melting point depression observed in binary mixtures. Melting temperatures as a function of conversion were obtained for both systems using differential scanning calorimetry together with size exclusion chromatography. In the case of the PBT-DGEBA-DDS system, a cloud-point curve was also obtained, showing an upper-critical-solution-temperature behaviour. On the basis of melting, cloud-point, vitrification and gelation curves, conversion-temperature transformation diagrams were generated for both systems. These diagrams can be used to design particular cure cycles to generate different morphologies in the phase separation process. In the case of PBT-DGEBA-MCDEA systems, PET could be either kept in solution in the matrix or separated by crystallization (initially or in the course of polymerization). For PBT-DGEBA-DDS systems, PET was always segregated from the matrix, either initially through crystallization or by attainment of the cloud-point curve in the course of reaction. Morphologies generated and resulting mechanical properties will be discussed in the second part of the series.