Dissolved acrylonitrile can be separated from a water stream by adsorption onto a polymeric sorbent (Dowex Optipore L-493) in a fixed bed and desorbed again by flushing the column with acetone. When the acetone fraction in the ternary mixture acrylonitrile/acetone/water is low and the acrylonitrile fraction is high, a phase separation between a water rich and an acrylonitrile rich phase occurs. In order to find the operating parameters for this separation process, the adsorbed amounts for all possible compositions of the bulk mixture need to be known. By use of a thermodynamic model, the loading in the ternary system was predicted from the three binary isotherms acrylonitrile/water, acrylonitrile/acetone, and acetone/water. These isotherms were determined experimentally. From the binary isotherms, the activity coefficients in the adsorbed phase were estimated. The binary model for the adsorbed phase activity coefficients was then extended to a ternary model. From this ternary activity coefficient model, the composition of the adsorbed phase was calculated as a function of the liquid phase composition. The agreement between the calculated adsorbed phase composition and the experimentally determined adsorbed phase composition is good to fair.