This contribution presents a chemical reactor model of a circulating fluidized bed (CFB) which takes into account adsorption effects and provides a description of the vertical profiles of gas concentration in the riser. The oxidation of NO is used as an example of a heterogeneous catalytic reaction. Mass transfer between the gas phase and solid particles in the CFB was investigated at various temperatures and solids mass flow rates. The measurements show that the mass transfer coefficient decreases with increasing solids mass flux and that the CFB resembles a bubbling fluidized bed as regards gas-solid contacting.