One of the prerequisites to the development of solvent gradient simulated moving bed (SG-SMB) chromatography for amino acid separation is to obtain their adsorption equilibrium data on a poly-4-vinylpyridine (PVP) resin under different contents of an additive solvent in the liquid phase. It was first found that ethanol could function well as a qualified additive solvent for the SG-SMB of interest. Then, the adsorption equilibria of 2-amino-3-phenyl-propanoic acid, 2-amino-3-(3-indolyl)-propanoic acid, and 2-amino-3-(4-hydroxyphenyl)-propanoic acid on the PVP resin were measured through a series of staircase frontal experiments while varying the ethanol content in the liquid phase from (0.0 to 0.2) volume fraction. On the basis of the resultant adsorption data, the partition coefficient, which is the ratio of solid-phase and liquid-phase concentrations at equilibrium, was determined for each amino acid. The determined partition coefficient showed a remarkable dependence on the ethanol content. Such a trend was successfully described with the Abel model, which correlated the partition coefficient as a function of the ethanol content. The adsorption data and the model parameters reported in this study will be of great value in the stage of designing the SG-SMB chromatographic process that is packed with the PVP resin and targeted at separating amino acids.