Abstract: Vapor liquid equilibria of mixed solvent/polymer ternary systems based on quartz spring weighing method were measured. In order to decrease the measuring error, 21 baskets containing polymer powder, one of them was hanged at the end of quartz spring to weigh the amount of mixed solvent absorbed by unit polymer sample, were used to increase the total amount of mixed solvent being adsorbed. The quantity of mixed solvent in vapor phase was calculated from the pVT data measurement of vapor phase. Both liquid and vapor phase composition were then determined by material balance. The vapor liquid equilibrium data for butanone/toluene/PMMA and butanone/acetone/PMMA systems at 308.15K have been measured. It is shown that the solubility of butanone in PMMA is enhanced due to the existence of toluene, while the solubility of toluene in PMMA will be lowered by adding butanone to the solution. The solubility behaviors of acetone and butanone in butanone/acetone/PMMA ternary system is similar to that in binary polymer solutions.