Experimental results are presented for the liquid-liquid equilibrium of a ternary system (N-vinylpyrrolidone + sodium sulfate + water) and three quaternary systems composed of (N-vinylpyrrolidone + poly(vinylpyrrolidone) + sodium sulfate + water) at 25 degrees C. The quaternary systems differ in the number averaged molecular mass of the poly(vinylpyrrolidones). That molecular mass was between about 4 000 and 140 000. The experimental results are described with a semi-empirical model (VERS model) for the excess Gibbs energy. (C) 2010 Elsevier B.V. All rights reserved.