The effects of pH on fumaric acid adsorption onto IRA900 ion exchange resin were studied. The optimum pH with the highest adsorption capacity depended on the fumarate concentration: the optimum pH was > 4.0 for fumarate concentrations < 5.0 g/L and < 3.0 for higher concentrations. Such different adsorption behaviors of IRA900 resins at different pHs were attributed to the different ion forms of fumarate present in solution at different pHs. Modeling results showed that the two-site-occupancy adsorption reaction between FA(2-) and resin active sites contributed to the high adsorption capacity at pH 5, whereas the intraparticle diffusion resulting from the hydrophobic interaction between H(2)FA and the hydrophobic resin backbone of IRA900 increased significantly and led to a higher adsorption capacity at pH 3.