The classical method for surface characterisation of the porous structure of the catalysts is nitrogen (N2) adsorption at -196 degrees C, which provides a catalyst surface area value valid for molecules similar in size to N2 (0.162?nm2/molecule). To complement and obtain more information about the materials, catalyst surfaces can be characterised using fractal geometry. The fractal dimension of a sample can be determined by the adsorption uptake of molecules of different sizes on the surface of interest in order to obtain a characteristic parameter of the surface geometry known as the fractal dimension, D. In this work, the value of D for a ?-alumina catalyst support has been determined (D?=?2.71 +/- 0.15) using different adsorbates (argon, nitrogen, isopropanol, pyridine, and n-butane). The decision process for choosing these adsorbates and the challenges of this type of characterisation method are discussed in this article. (C) 2011 Canadian Society for Chemical Engineering