An X-ray reflectivity study of the silicon isobutane interface is presented. Thin isobutane films were adsorbed on a silicon wafer and their film thickness was measured as a function of pressure in order to determine the effective Hamaker constant which is a proportion of the coupling between substrate and adsorbed film. A comparison with theoretical expressions of the effective Hamaker constant is given. (c) 2006 Elsevier B.V. All rights reserved.