The adsorption and desorption of 6-aminopenicillanic acid (6-APA) in aqueous solution has been studied using activated carbon as the adsorbent. The extent of adsorption was found to tie strongly dependent on the aqueous phase pH and this dependence could be interpreted from a model for neutral species adsorption. Desorptionstudies suggest that a small fraction of 6-APA adsorbs irreversibly on activated carbon. Adsorption equilibrium data were correlated with the Langmuir, Freundlich and Redlich-Peterson expressions with the Langmuir model being found to provide the best fit of the experimental data. The rates of adsorption and desorption appear to follow the first-order kinetics under the experimental conditions used in the study. However, more precise rate expressions requires the inclusion of the external mass transfer effect initially and the particle diffusion effect at the later stage. Adsorption enthalpy calculated from the Van't Hoff plot was found to be 3.92 kcal mol(-1). (C) 1997 Elsevier Science B.V.