Vapor - liquid equilibria for mixtures of acetone and poly butyl methacrylate with three different molar masses from 17278 g/mol to 88059 g/mol were obtained at 25. 0℃ by using the Quartz - Spring gravimetric method. The reduced pressure p1/p°1 versus mass fraction W1 plots were found to be independent of the molar mass of the polymer within experimental error. The calculated Flory - Huggins interaction parameters X exhibit a strong concentration dependence. The molar mass of the polymer also exerted a definite influence. The experimental Flory - Huggings interaction parameters were correlated by the revised Freed model.