Water activity data of several binary water + polymer systems were determined experimentally using both vapor-pressure osmometry (VPO) and improved isopiestic methods. The polymers were poly(ethylene glycol) dimethyl ether 250, poly(ethylene glycol) dimethyl ether 500, poly(ethylene glycol) dimethyl ether 2000, poly(ethylene glycol) monomethyl ether 350, poly(ethylene glycol) 200, poly(ethylene glycol) 6000, and poly(propylene glycol) 400. The obtained water activities data were used to calculate the vapor pressure of solutions as a function of concentration. Water activity of all investigated solutions increased with increasing temperature and polymer molar mass. A comparison is given between the vapor liquid equilibrium data inferred from VPO and isopiestic methods, for different aqueous polymer solutions at 298.15 K, and there is a good agreement between the results of the investigated methods. Furthermore, the segment-based local composition models, nonrandom two-liquid (NRTL), Wilson, and universal quasichemical (UNIQUAC), were used to correlate the experimental activity data.