The adsorption of humic acid (HA) onto nano-structured TiO2 microsphere surface is rather a complex process that depends on the nature of the adsorbent surface, the nature of adsorbate and the solution condition. This study was undertaken to model the adsorption behavior of HA onto TiO2 microsphere and to elucidate the major interaction mechanisms. The adsorption of HA onto TiO2 microsphere was observed to be well fitted by Langmuir-Freundlich (L-F) model at any solution condition. Zeta potential analysis revealed that the adsorption was initiated by electrostatic interaction and followed by ligand exchange which eventually led to the formation of surface complexation. XPS analysis further confirmed the formation of surface complex on adsorbed TiO2 microsphere. The results suggested that the oxygen atom of the carboxylic group of HA was bounded to the Ti4+ cations. While the Ca2+ cation was not directly intact with TiO2 microsphere surface but also bounded through the oxygen in carboxylic group onto the Ti4+ cations on the surface of TiO2 microsphere. (C) 2011 Elsevier B.V. All rights reserved.