The objective of this work was to improve understanding of the mass transfer between gas and solids in the riser section of a gas-solids circulating fluidized bed (CFB). In order to experimentally investigate gas-solids mass transfer in a CFB, a suitable low-temperature mass transfer process was applied: the decomposition of ozone (O-3) using particles as catalyst. Particles used were angular cast steel with an average diameter (mass mean) of 134 mum. The experiments were carried out in a semi-industrial sized CFB. Operating conditions were varied, with superficial gas velocity of up to 7.2ms(-1) at 60 degreesC riser temperature, and riser net solids mass flux up to 53 kgs(-1)m(2). The overall solids concentration was around 2-2.5% and about 10% in the lower part of the riser. Ozone concentration profiles, axial pressure profiles, solids net mass flux as well as local particle velocities were measured. Gas velocity profiles were assumed in order to calculate the total transport of ozone at different heights in the riser. The importance of the lower part of the riser with respect to mass transfer is shown.