Isopiestic measurements were used to obtain activities for aqueous solutions of poly(ethylene glycol) dimethyl ether 2000 + poly(propylene glycol) 400 and their corresponding binary aqueous solutions at temperatures T = (298.15, 308.15, and 318.15) K. The data obtained were used to calculate the vapor pressure of solution as a function of concentration. The effect of temperature on the vapor liquid equilibrium (VLE) of binary and ternary systems has been studied. Furthermore, the activity results for binary and ternary aqueous solutions were correlated to the Flory-Huggins and Wilson models.