The compatibility of PSSNa and PVSNa in aqueous solution has been studied. PSSNa and PVSNa are compatible when the total concentration is less than 1.0 mol/l, but phase separation occurs when the total concentration exceeds 1.0 mol/l. The cloud point curve is of a typical UCST type. Addition of salt causes a shift of the boundary toward a lower concentration. It may be considered that these new critical phenomena are a reflection of the balance between electrostatic and non-electrostatic interactions. The time evolving processes of the phase separation has also been investigated. The characteristic behavior of spinodal decomposition is observed when the temperature of a mixed solution is jumped down passing through the critical point. (C) 2000 Elsevier Science Ltd. All rights reserved.