Cloud-point curves were determined for blends of a rubber (butadiene-acrylonitrile random copolymer terminated in non-functional groups, NFBN) and a cyanate ester (4,4’-dicyanate-1,1’-diphenylethane, CE). Measurements were made in the initial blend and in partially polycondensed blends (thermal polycyclotrimerization of the CE monomer). Cloud-point conversions and temperatures were determined for different rubber fractions in the initial formulation. A thermodynamic analysis based on a Flory-Huggins equation, taking polydispersity of both components into account, led to the following conclusions. (1) Phase separation was the result of the change in two different contributions to the free energy of mixing : (a) a decrease in the entropic contribution due to the increase in the oligomer size; (b) a decrease in the enthalpic contribution as a result of the decrease in the cohesive energy density of the CE-oligomer in the course of polycondensation, (2) Spinodal demixing was excluded as a possible mechanism of phase separation during polycondensation in solutions containing less than 12% rubber by volume.