T his paper presents the results of a gas mixing study carried out in a laboratory scale (I.D. = 0.23 m, height = 7.6 m) circulating fluidized bed with secondary air (SA) injection. Steady-state tracer gas experiments were performed to investigate the effects of SA injector design on radial gas dispersion under varying operating conditions (U-o = 3, 5 ms(-1); 0 < G(s) < 25 kg m(-2) s(-1)) with silica sand particles (rho(s) = 2650 kg m(-3), d(p) = 250 mum). Three different types of SA injectors that could feed the SA to the riser at different orientations were tested in the experiments; tangential, radial and mixed. Axial pressure measurements show that the tangential SA injector considerably increases the solids hold-up in the riser compared with non-SA operation at the same superficial gas velocity and solids circulation rate. Tracer gas measurements and calculated dispersion coefficients indicate that SA injection significantly increases the radial gas dispersion regardless of the design of the injector. The most effective injector in improving gas mixing was found to be the radial injector. This IS ttributed to the large-scale fluctuations caused by impinging radial SA jets. Furthermore, the mixing pattern of SA with rising gas-solid flow shows considerable differences with different injector types.