Ellipsometric porosimetry is a valuable tool to determine gas loading of porous materials. Usually the Lorentz-Lorenz effective medium theory is used, instead of the more accurate Bruggeman theory. In contrast to Lorentz-Lorenz, the Bruggeman model requires detailed knowledge on the constituents of the porous material. A first order perturbation of both effective medium approximations is used to analyze the difference between these models. Similar results are only found for materials with 70% porosity. Below 50% porosity, the gas load is underestimated with the Lorentz-Lorenz model. For porous silica and alumina with 50% porosity, the use of Lorentz-Lorenz leads to a systematic error of 18% of the load capacity. (C) 2010 Elsevier B.V. All rights reserved.