Vapor pressure data of six binary acetonitrile + polymer systems were determined experimentally using an improved isopiestic method at 298.15 K. Polymers were poly( ethylene glycol) 200, poly( ethylene glycol) 6000, poly( propylene glycol) 425, poly( propylene glycol) 1000, poly( vinylpyrrolidone) 10000, and poly( ethylene glycol) methaacrylate 360. Sodium iodide was used as the isopiestic standard for the determination of vapor pressures. The activities of acetonitrile in the polymer solutions were calculated from the vapor pressure data. Furthermore, the segmentbased local composition models, NRTL, Wilson, NRF- NRTL, NRF- Wilson, and UNIQUAC were used to correlate the experimental vapor pressure and activity data.