The underlying objective of the present study is to increase gas-solids contact in a circulating fluidized bed by the introduction of obstacles in the riser portion. The presence of such obstacles leads to suppression of radial inhomogeneities in the solids mass flux and concentration, and break-up of solids clusters. At ambient conditions, gas-solids mass transfer was investigated for cocurrent upward flow of air and microsize solid particles (FCC, 70 Am diameter) over a regularly structured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross-sectional area of 0.06 X 0.06 m2, and contains regularly stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas-solids contact. Gas mass fluxes used were 1.4 and 2.7 kg m-2 s-1. Solids mass fluxes were varied in the range 0 less-than-or-equal G(s) less-than-or-equal-to 12 kg m-2 s-1. Experimental mass transfer data were obtained by applying the method of adsorption of naphthalene vapor on FCC particles. A conservative estimate of the apparent gas-solids mass transfer coefficient k(g)* could be derived from the naphthalene vapor concentration profile along the packed section on the basis of a plug-flow-model interpretation, while assuming single-particle behaviour and neglecting intraparticle diffusion effects. Such k(g)* values appear to increase with increasing gas mass flux, but decrease with increasing solids mass flux (and consequently increasing solids volume fraction) probably due to the corresponding increase in particle shielding. Comparison of the present results with available literature data for similar solid materials suggests that the effect of the packing inserted into the CFB is significant : the Sherwood numbers derived from the present study are relatively high.