In a downer reactor (0.1 m-I.D.x3.5 m-high), the effects of gas velocity (1.6-4.5 m/s), solids circulation rate (0-40 kg/m(2)s) and particle size (84, 164 mu m) on the gas mixing coefficient have been determined. The radial dispersion coefficient (D-r) decreases and the radial Peclet number (Pe(r)) increases as gas velocity increases. At lower gas velocities, D-r in the bed of particles is lower than that of gas flow only, but the reverse trend is observed at higher gas velocities. Gas mixing in the reactor of smaller particle size varies significantly with gas velocity, whereas gas mixing varies smoothly in the reactor of larger particle size. At lower gas velocities, D-r increases with increasing solids circulation rate (G(s)), however, D-r decreases with increasing G(s) at higher gas velocities. Based on the obtained D-r values, the downer reactor is found to be a good gas-solids contacting reactor having good radial gas mixing.