The dispersion of a Pt/γ-Al2O3 catalyst was investigated by various experimental techniques. The values obtained by gas-phase hydrogen and carbon monoxide adsorption and transmission electron microscopy (TEM) were consistent for the metal:adsorbate stoichiometry of 1:1. However, these methods overestimated the number of active sites available for a liquid-phase reaction. For the hydrogenation of styrene in the presence of CS2 as a catalyst poison, only the defect sites were found to be active in the catalytic reaction. The defect sites were related to the initial coverage dependent region of the adsorption heats measured by gas-phase microcalorimetry. © 2009 Elsevier B.V. All rights reserved.