The adsorption of the amino acids leucine, phenylalanine, and tryptophan and, for a comparison, of the antibiotic penicillin-G is studied for a range of porous polymeric adsorbents. The adsorbent materials investigated include both highly hydrophobic adsorbents as well as functionalized, more hydrophilic ones. The adsorption isotherms approach the ideal Langmuir form. For the amino acids considered in this work, the isotherms are essentially independent of pH, but they vary significantly with temperature and with the addition of alcohols. Heats of adsorption for phenylalanine are obtained from van’t Hoff plots of the Henry’s law constant limit of the Langmuir isotherm and they are found to be larger for the more hydrophobic adsorbents. However, adsorption affinities for this amino acid are larger for the more hydrophilic sorbents, indicating a significant entropic contribution for the adsorption on these materials.