We studied the effect of the intercalant-host interaction in homogeneous a-LixWO3 by electrochemical experiments in films of different thickness (100-400 nm) prepared by electron beam evaporation. The intercalation thermodynamics was derived from the analysis of the chemical capacitance (proportional to the derivative of the composition x with respect of the chemical potential, partial derivative x/partial derivative mu). It was found that thinner films can be modeled by the ideal statistics of a dilute solution (non-interacting lattice gas), while host-guest interactions are the determining factor for the electrode performance in case of thicker films (>200 nm). (c) 2005 Elsevier B.V. All rights reserved.