AbstractCastor oil (CO) was used to replace polydisperse commercial rubbers (carboxy‐ or epoxy‐terminated butadiene‐acrylonitrile random copolymers, CTBN or ETBN) in model systems developed to analyse the origin of the phase separation process in rubber‐modified thermosets. Mixtures of CO with an epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) showed a higher miscibility than a typical CTBN (18% acrylonitrile)‐DGEBA system. Thermodynamic factors accounting for this behaviour are discussed using the Flory‐Huggins lattice model. Addition of a stoichiometric amount of ethylenediamine (EDA) to a DGEBA‐CO solution increased the miscibility. A pseudo‐binary approach was used to fit the cloud point curve up to 30% volume fraction of CO. Percentage conversions at the cloud point during the DGEBA‐EDA polymerization were experimentally determined and compared with theoretical predictions using the pseudo‐binary approach in the framework of the Flory‐Huggins lattice model. Reasonable agreement was found, giving direct evidence of the fact that phase separation results from the decrease in the entropic contribution to the free energy of mixing