Cloud-point measurements were made on quasi-binary mixtures of diglycidyl ether of bisphenol A (DGEBA) epoxy resin with poly(ether sulfone) (PES). The relative molecular masses of both resin and polymer were varied, and temperature-dependent Flory-Huggins interaction parameters were estimated on the assumption that both components are monodisperse. The data were used to calculate lower critical solution temperatures (LCST) and compositions, and to construct coexistence curves. The evolution of two-phase morphology during curing of epoxy-PES blends containing 4,4’-diaminodiphenylmethane (DDM) hardener is related to these results.