The activities of methanol, ethanol, 2-propanol, and 1-butanol in poly(propylene glycol) (PPG) (M-p: 1000) solutions have been measured by the isopiestic method at 25 degreesC. Sodium iodide and calcium chloride were used as the isopiestic standards for the calculation of activities. The original equation of Flory-Huggins and the modified Flory-Huggins equation with concentration dependent interaction parameters have been used for the correlation of obtained experimental solvent activity data. Better agreement with the experimental data was obtained using the modified Flory-Huggins equation.