Polymerization of styrene (St) and acrylonitrile (AN) in the presence of polypropylene glycol (PPG) is a heterogeneous process, involving the phase equilibria of two quaternary systems (St-AN-poly(St-co-AN) (SAN)-PPG), i.e. PPG-rich phase and SAN-rich phase. Partition behaviors of St and AN between each phase were investigated by modeling and experiments. A phase equilibrium model based on Flory-Huggins polymer solution theory was established. Temperature-dependent Flory-Huggins interaction parameters (FHIPs) were estimated from Hildebrand-Scatchard or Hansen three-dimensional solubility parameters. The validity of the estimated FHIPs and the good predictive capability of the model were demonstrated by comparing the calculated results with the experimental data. The mean deviation is around 3%. The influences of various conditions on partition behaviors of St and AN were studied experimentally, and well described by the model. It is found that the miscibility between SAN and PPG is very limited, and monomers are prone to exist in PPG-rich phase, and partition coefficients of St and AN are significantly affected by St/AN ratio and temperature, especially by St/AN ratio. (c) 2012 Elsevier B.V. All rights reserved.