In this study the solubility of amino acids in water for various pH values at 298.15 K is investigated. A colorimetric method using a spectrophotometer was employed to measure solubility data for the five amino acids, DL-alanine, L-leucine, L-isoleucine, L-serine and DL-phenylalanine. in water. The pH effect, changing from 2 to 10 adjusted by the addition of HCl or NaOH, has a significant effect on the solubility data. This is due to the fact that amino acids appear in several forms in solution, depending on the pH, which are neutral zwitterions, cationic and anionic species. These are not only different forms in chemical equilibrium with each other. but also with hydrogen ions in an aqueous solution. To model the solubility of amino acids in water, these chemical equilibria were considered. An NRTL activity coefficient model was used to describe the non-idealities in the solution. The calculated results agree well with the experimental data. (C) 2009 Elsevier B.V. All rights reserved.