Based on measurements in a circulating fluidized test unit with a riser of 0.4 m i.d., a two-dimensional two-phase model for gas mixing has been developed. Radial gas dispersion and gas backmixing caused by dense clusters falling countercurrently to the main now of a lean gas/solid suspension are considered. The model has successfully been compared with experimental data over a wide range of operating conditions. The model accounts for the main mixing phenomena and may be applied to calculations of chemical reactions in CFB risers.