The partitioning of iodide was studied in an aqueous biphasic system (ABS) comprised of ammonium sulfate, polyethylene glycol (PEG), and water at 25 degrees C. The average molecular weights of PEG were (2078, 3819, and 9887) g center dot mol(-1). In a prior determination, data for the liquid-liquid equilibrium of this system were obtained and correlated with the nonrandom, two-liquid (NRTL) thermodynamic model. Data on the partition coefficients and percentage extraction of iodide were obtained , which were correlated as a function of the concentration gradient of the PEG between the phases and the average molecular weight of the PEG. The fit equation obtained had a mean deviation of 0.0171 and a mean error of 1.89 %.