The effective medium theory (EMT), describing diffusion in random composite media, was tested for its suitability to be used in the estimation of the effective diffusion coefficient of water in the inhomogeneous open zeolite 4A coatings prepared by them substrate heating method. The solid zeolite matrix and the void regions distributed in the matrix were taken as the two different phases in the system. The diffusion coefficient values obtained were compared to those determined in a previous study by the help of an experimental method involving the coupled use of thermogravimetric analysis and mathematical modeling. The estimated diffusivities, which increased with increasing void fraction, were in good agreement with the experimentally determined values. (C) 2004 Elsevier B.V. All rights reserved.