AbstractAn analysis is made of the thermodynamic and kinetic factors affecting phase separation in blends of di‐, tri‐ and tetrafunctional epoxy resins, cured with either diamino‐diphenylsulphone (DOS) or with dicyanodiamide (Dicy), and modified with either poly(ethersulphone) (PES) or carboxyl‐terminated butadiene acrylonitrile (CTBN) rubber. The applicability and limitations of the solubility parameter approach in predicting compatibility in reactive blends are con