The osmotic coefficients and water activities in binary solutions of water-soluble DMAM-TBAM copolymer (the abbreviation for N,N-dimethylacrylamide and t-butylacrylamide copolymer) with different molar masses were determined by isopiestic method using NaCl(aq) as the reference standard at 308.15 K. From these measurements, values of the vapor pressure of solutions were determined, and the effect of molar mass on the vapor-liquid equilibrium of DMAM-TBAM copolymer has been studied. The experimental water activities data have been correlated using the Flory-Huggins equation, the modified Flory-Huggins equation, and the Freed Flory-Huggins equation + the nonrandom two-liquid model (NRTL). Better agreement with the experimental data was obtained using the Freed Flory-Huggins equation + NRTL model.