The adsorption equilibria necessary for the design of a chromatographic process for amino acids separation were measured on a Capcell Pak C-18 resin using a dynamic frontal analysis method. The single-component adsorption data of 2-amino-3-phenyl-propanoic acid, 2-amino-3-(3-indolyl)-propanoic acid, and 2-amino-3-(4-hydroxyphenyl)-propanoic acid were obtained at room temperature while varying the methanol content in the mobile phase from 5 % (v/v) to 70 % (v/v). On the basis of the resultant adsorption data, the Henry's constant, which is the ratio of solid-phase and liquid-phase concentrations at equilibrium, was determined for each amino acid and each mobile-phase composition. There was a large difference in Henry's constant among the three amino acids investigated. In addition, the Henry's constant of each amino acid showed a significant dependence on the mobile-phase composition. To correlate such a behavior, several models were tested. It was found that Abel's model containing three parameters was the most successful in correlating the Henry's constant of each amino acid as a function of mobile-phase composition. The data and model parameters reported in this study can be of use to the design of a chromatographic separation process based on a solvent gradient operation mode.