The adsorption of L-histidine (L-His) and DL-tryptophan (DL-Trp) on cholesterol, silica, alumina, and graphite surfaces from aqueous solution at pH 9.2 has been studied in the temperature range of 25 to 50 degrees C. The data have been analyzed, wherever applicable, by Langmuir and Hill equations to obtain (1) the monolayer adsorption capacity (x(m)) and the equilibrium constant (k') for the adsorption and desorption processes and (2) the Hill coefficient (n) at different temperatures. The results of adsorption on different kinds of adsorbents have been compared. The energetic parameters for the adsorption process have also been evaluated and compared. Copyright 2001 Academic Press.