A virial theory was used to relate molecular interactions (in terms of second virial coefficients, B') and molecular size ratios to liquid–liquid phase separation. Application of the virial theory to binary hard sphere mixtures (additive and non-additive) confirmed the applicability of this simple approach towards predicting phase behavior based on two-particle interactions. Experimentally, second cross virial coefficients were obtained for dextran/gelatin, whey protein isolate (WPI)/gelatin mixtures and whey protein aggregate (WPA)/gelatin mixtures using membrane osmometry at varying ionic strength. From this, solvent conditions where interactions between proteins are dominated by electrostatics and solvent conditions where interactions are dominated by hard body interactions could be determined. Using experimentally obtained second virial coefficients, the liquid–liquid phase separation for gelatin/dextran mixtures was successfully predicted. Second cross virial coefficients for gelatin/whey protein isolate and for gelatin/whey protein aggregate could be related to the absence of phase separation in these mixtures. This could be related to a similar size of the proteins and their non-additive behavior at conditions where they mainly interact via hard body interactions.