Epoxy-aromatic diamine formulations are simultaneously modified with two immiscible thermoplastics (TPs), poly(ether imide) (PEI) and polysulfone (PSF). The epoxy monomer is based on diglycidyl ether of bisphenol A and the aromatic diamines (ADS) are either 4,4'-diaminodiphenylsulfone or 4,4'-methylenebis(3-chloro 2,6-diethylaniline). The influence of the TPs on the epoxy-amine kinetics is investigated. It is found that PSF can act as a catalyst. The presence of the TP provokes an increase of the gel times. Cloud-point curves (temperature vs. composition) are shown for epoxy/PSF/PEI and epoxy/PSF/PEI/AD initial mixtures. Phase separation conversions are reported for the reactive mixtures with various TP contents and PSF/PEI proportions. On the basis of phase separation and gelation curves, conversion-composition phase diagrams at constant temperature are generated for both systems. These diagrams can be used to design particular cure cycles to generate different morphologies during the phase separation process, which is discussed in the second part of this series. (C) 2004 Wiley Periodicals, Inc.