Equilibria for adsorption of L-glutamic acid on a general weakly basic ion exchanger which has only one kind of fixed ammonium group are investigated theoretically and experimentally. The experimental equilibrium isotherm for adsorption of L-glutamic acid on the commercial weakly basic ion exchanger was independent of the initial concentration of L-glutamic acid but depended on pH of the solution significantly. The significant effect of pH in the experimental equilibrium isotherm disappeared in the plots of equilibrium amount of L-glutamic acid in the resin phase vs the concentration of the neutral L-glutamic acid in the liquid phase. This suggested that the adsorption of L-glutamic acid was controlled by the acid/base neutralization reaction between neutral L-glutamic acid and fixed ammonium group of the weakly basic ion exchanger. Theoretical equations for equilibrium isotherm for adsorption of L-glutamic acid on the weakly basic resin was derived by assuming acid/base neutralization reaction between carboxylic group of the neutral L-glutamic acid and the fixed ammonium group of resin. The experimental equilibrium isotherms for any constant pH and the effect of pH on the equilibrium amount of L-glutamic acid adsorbed on the weakly basic resin were determined well from the theoretical equations.