Adsorption of L-glutamic acid on the polyaminated highly porous chitosan (PEI-CH) appeared technically feasible. The saturation capacity for adsorption of L-glutamic acid was about 1.43 times larger than a commercial weakly basic ion exchanger DIAION WA30. The experimental equilibrium isotherm (q(A)-C-TA curve) for adsorption of L-glutamic acid on PEI-CH was independent of the initial concentration of L-glutamic acid but depended on pH of the solution significantly. It appeared that the adsorption of L-glutamic acid was controlled by the acid/ base neutralization reaction between neutral L-glutamic acid (zwitterion, A(+/-)) and the weakly basic ion exchanger. The significant effect of pH on the q(A)-C-TA curve disappeared in the q(A)-C-A+/- curve. Theoretical equations for equilibrium isotherms for adsorption of L-glutamic acid on PEI-CH were derived by assuming neutralization reaction between the carboxylic group of the neutral L-glutamic acid and each three different fixed amino groups of PEI-CH, The theoretical equations correlated the experimental equilibrium isotherms well.