A steady-state dispersion model was used to determine thr radial gas mixing coefficients D-r in the dilute region of a cold fast fluidized bed, 0.1 m.i.d. and 4 m high. CO2 used as tracer was injected into the centre of the bed from a point source. Radial concentration profiles of tracer gas were measured in two planes downstream of the injection point for a broad range of air velocities and solid fluxes, using two sizes of sand particles of 710 and 380 mu m. A systematic experiment was carried out to find out the main variables acting on the radial gas mixing in the dilute region of a fast fluidized bed. An analytical solution derived by Towle and Sherwood for the description of radial gas mixing in turbulent single-phase flow was applied to determine the radial dispersion coefficients, which were found to be dependent on the superficial gas velocity and solid circulating flux. The D-r values were well correlated with an apparent suspension Reynolds number (Re) by an equation of the type D-r = aRe(b). The proposed equation allows the radial gas coefficient to be predicted as a function of the air velocity and external solid flux present in the riser. This equation, with its corresponding parameters, is applied to the results of other authors and an acceptable fit was found. The high Pe numbers (500-2000) obtained in the dilute region of the fast fluidized bed indicate that the flow of the gas in the dilute region approximates to plug flow.