Catalytic ozone decomposition reaction was used to study the performance of a 76 mm i.d. and 5.8 m high gas-solid circulating fluidized bed (CFB) downer reactor. Optical fiber probes and an ultraviolet (UV) ozone analyzer were used to obtain comprehensive information about local solids holdup and ozone concentration profiles at different axial and radial positions at superficial gas velocity of 2-5 m/s and solids circulation rates of 50 and 100 kg/m(2)s. Axial ozone concentration profiles significantly deviated from the plug-flow behavior, with most conversion occuring in the entrance region or flow developing zone of the downer reactor. Strong correlation was observed between the spatial distributions of solids and extent of reaction; high local solids holdups cause lower ozone concentrations due to higher reaction rates. Radial gradients of the reactant (ozone) concentrations increased in the middle section of the downer, and decreased with increasing superficial gas velocity and solids circulation rate. Contact efficiency, a measure of the interaction between gas and solids indicated high efficiency in the flow developing zone and decreased with height in the fully developed region. (C) 2011 Elsevier Ltd. All rights reserved.