This paper documents a continuation of work published in Langmuir 2005, 21, 8743-8750. We report new aspects of this amino acid adsorption study, including the effect of changing zeolite framework type and results from adsorption of mixed amino acids. In single-amino-acid adsorption experiments, zeolite Y (Si/Al = 2.5) was found to adsorb no phenylalanine while admitting arginine to a similar extent as observed with zeolite,. Using this zeolite Y, we measured mixed-amino-acid separation selectivities for arginine/phenylalanine as large as 25 000 or 2 orders of magnitude larger than the corresponding selectivities measured with zeolite,.