The gas backmixing characteristics in a circulating fluidized bed (0.1 m i.d., 5.3 m high) of fluid catalytic cracking (FCC) particles have been determined. The gas backmixing coefficient has been determined based on the axial dispersion model by using two tracer gases (He and CO2) in a circulating fluidized bed (CFB). The obtained gas backmixing coefficients by using the adsorbed CO2 gas overestimates the gas backmixing coefficient 60-62% in the dense region, 49-62% in the transition region and 2.4-19% in the dilute region compared to the values obtained by the non-adsorbed He tracer gas. The gas backmixing coefficient increases with increasing the ratio of particle to gas velocities (U-p/U-g). In the upper dilute region, the core annulus model is proposed to determine the gas backmixing coefficients and the mass transfer coefficients between the core and the annulus regions. The gas backmixing coefficient and mass transfer coefficient increase with increasing U-p/U-g. (C) 1998 Elsevier Science S.A. All rights reserved.