The characteristics of gas-solids mass transfer were studied in a downer reactor with the adsorption Of CO2 tracer by activated charcoal particle. Axial distribution profiles of the CO, tracer concentration were measured under several different operating conditions. Mass transfer coefficient and axial dispersion coefficient were evaluated with a non-linear regressing method (Marquardt method). It was found that the operating conditions such as solids circulation rate and gas velocity have complicated effects on the gas-solids mass transfer coefficient, and the effect of superficial gas velocity seemed to be more significant than that of solids circulation rate. The following empirical correlations were found to best fit the experimental data for axial dispersion coefficient, E. and overall mass transfer coefficient, K-F: Pe(1) = Lu-g/E-a = 0.226Re(0.499)m(0.302) Pe(2) = u(g)/K(F)aL = 0.000185Re(0.618)m(-0.983) with m=G(s)/(u(g)rho(p)) = nu(s)(1-epsilon)/(u(g)epsilon). (c) 2007 Elsevier B.V. All rights reserved.