Chitosan ultrathin films have been formed on polycrystalline Au substrates using the LbL technique with the purpose of studying its interaction with bovine beta-lactoglobulin (beta-LG) at the solid-liquid Interface The immobilization of chitosan was followed by Quartz Crystal Microbalance with energy dissipation (QCM-D) Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) The behavior of the chitosan films in the presence of beta-LG solutions with different bulk concentrations ([beta-LG]) ionic strength (I) and pH has been investigated using the same techniques plus Atomic Ewe Microscopy (AFM) The results showed that for pHs lower than protein s pI weak intermolecular forces (H bonding Van del Waals hydrophobic etc) are established between beta-LG and chitosan (especially close to the pI) leading to low coverage nonspecific adsorption On the contrary when pH > pI strong ionic bonding through attractive electrostatic interactions lead to high coverage adsorbed phases composed of large beta-LG aggregates The adsorption process was shown to consist of a relatively fast step (in which these interactions are predominant) which is followed once the beta-LG monolayer is exceeded by the slow formation of thicker and increasingly viscoelastic films through beta-LG self-aggregation QCM-D and AFM experiments unveiled the role of [beta-LG] and I on the formation of these aggregates The adsorption isotherm built from impedance data in the medium-low [beta-LG] range (0 001-0 3 mg m L-1) showed good fitting to the Langmuir model confirming that the formation of one beta-LG monolayer is achieved in this concentration range (C) 2010 Elsevier Ltd All rights reserved