The phase separation of a rubber in the course of a thermosetting polymerization is associated with heat release. Mixing is endothermic as revealed by the positive value of the interaction parameter while demixing is exothermic. Differential scanning calorimetry may be used to detect heat effects related to phase separation. Experimental results are reported for an unreactive system (castor oil/epoxy resin) and reactive systems (modifier/epoxy resin/diamine) using either castor oil or epoxy-terminated copolymers of acrylonitrile and butadiene as modifiers.