The adsorption of pretreated palm oil mill effluent (POME) onto a granular activated carbon (GAC) fixed bed has been studied using the incorporation of ideal adsorbed solution theory (IAST) into the homogeneous surface diffusion model (HSDM). The pretreated POME is a complex solution of a ternary system containing three major organic solutes of carbohydrate constituents, protein, and ammoniacal nitrogen. The incorporation of IAST into HSDM allows multicomponent adsorption predictions using single-solute equilibrium isortherms. Batch adsorption and fixed-bed adsorption were conducted to obtain the experimental data for a multicomponent adsorption system using pretreated POME. The residence time in the bed was varied by manipulating the bed length and the feed flowrate. On the basis of the comparison of the experimental breakthrough curves with the simulation results, IAST gives a good prediction of multicomponent adsorption on GAC when it is incorporated into HSDM.