The accessible metallic area measured by hydrogen chemisorption on model ceria-alumina supported platinum or(and) rhodium catalysts has been compared to the results obtained in cyclohexane aromatization activity tests. The benzene formation was studied at 573 K after reduction by hydrogen at the same temperature. The turnover frequency, calculated on the basis of hydrogen chemisorption data, was found to depend on the type of support, ceria or alumina, in interaction with the precious metal particles. However, for the catalysts supported on ceria-alumina, the activity per metallic platinum or rhodium site was almost the same for both fresh catalysts or after hydrothermal aging at 1273 K. The possible use of the cyclohexane aromatization activity measurement for the determination of the metallic area of three-way catalysts is examined. Compared to hydrogen chemisorption, this method presents the advantage of a better sensitivity for measuring very low dispersions of aged catalysts, typically dispersions inferior to 3%. In the discussion, the possible influence of hydrogen spill-over effects and of the connected role of chlorine are considered. Although not directly studied, the possible effect of sulphur present in commercial catalysts aged in vehicles is also discussed.