VLE data for mixed solvents-polymer systems are rare in literature. The pivotal problem in the measurement is how to accurately determine compositions of mixed solvents. In this work, a new experimental method is presented by combining quartz spring weighing and material balance. To improve the accuracy, 21 baskets containing polymer powder are used to increase the total amount of mixed solvents being adsorbed. One of these baskets is used as a monitor, while the weight of the others can be estimated. The quantity of mixed solvents in vapor phase is calculated from the pVT data measured simultaneously. Both liquid and vapor compositions are then determined by material balance, VLE data for four mixed solvents-polymer ternary systems. butanone-toluene-poly(methyl methacrylate)(PMMA), butanone-acetone-PMMA, benzene-toluene-polystyrene(PS) and benzene-toluene-PMMA at 308.15 K, have been measured. The solubility of butanone in PMMA is enhanced due to the existence of toluene, while the solubility of toluene in PMMA is lowered due to the existence of butanone. The solubilities of acetone and butanone in butanone-acetone-PMMA ternary system, as well as those of benzene and toluene in benzene-toluene-PS ternary system, are consistent to those in the corresponding binary polymer solutions. The results were correlated by an equation of state developed previously for polymer systems.