[N-(Phosphonomethyl)imino]diacetic acid (PMIDA) and iminodiacetic acid (IDA) are two structurally similar amino acid derivatives found prevalently in a highly concentrated NaCl solution in the waste stream from the synthesis of the herbicide Roundup. Because IDA is biodegradable and PMIDA is not, it is important to be able to separate the two acids. In this work, it is shown that PMIDA and IDA have very different affinities for the basic polymer adsorbent poly(4-vinylpyridine), while NaCl; has a negligible affinity. The adsorbent has been found to be robust as well as chemically and thermally regenerable. Single- and multiple-frontal column tests are used to determine equilibrium adsorption isotherms and mass-transfer parameters at temperatures between 23 and 85 degrees C. An Arrhenius temperature dependence is used to correlate the equilibrium adsorption data. A parallel pore and surface diffusion model can accurately describe the breakthrough and elution behavior of the high-affinity solutes, PMIDA and IDA, while a pore diffusion model can accurately describe the behavior of the nonretained solute NaCl. The models can accurately describe breakthrough behavior at temperatures between 23 and 85 degrees C. The data obtained in this work form the backbone for subsequent studies on batch and continuous processes for separating NaCl, IDA, and PMIDA.