Gas-solids contacting in risers has been studied based on measurements of mass transfer controlled CO oxidation over a Pt/gamma-alumina catalyst, and on experimental results published by Ouyang et al. (1995) for the kinetically controlled ozone decomposition. In the present experiments, the catalyst activity was varied by mixing the active catalyst particles with similar, but inert gamma-alumina particles (in ratios from 150 to 2500 m(inert)(3)/m(cat)())(3), whereas Ouyang and co-workers varied the operating temperature (300 to 350 K). Mass transfer controlled CO oxidation occurs at temperatures above 750 K. A negative square root dependency has been observed for the relationship between the Sherwood number and the solid hold-up. Increasing the gas velocity always improves the gas-solids contacting. The local catalyst activity appears to be an important parameter. As an important conclusion of the present work, it can be stated that at a high local activity, the conversion rate per unit volume of catalyst decreases significantly due to local depletion of reactant.