AbstractGas mixing in a spout‐fluid bed with a cross section of 0.3 × 0.03 m and height of 2 m was investigated by simultaneously injecting two different tracer gases. One was injected into the spouting gas flow and the other was injected into the fluidizing gas flow. Steady‐state tracer gas measurements were carried out to obtained radial tracer gas concentrations at various bed elevations. Effects of two important operating parameters—spouting gas velocity and fluidizing gas flow rate—on the gas mixing were discussed with flow patterns recorded by a high‐resolution digital CCD camera. The results show that increasing spouting gas velocity and fluidizing gas flow rate can both promote the gas mixing in spout‐fluid beds. Increasing fluidizing gas flow rate is the more effective way, given that a satisfactory mixing condition at a relatively low bed height can be obtained by increasing the fluidizing gas flow rate. For both cases, it is difficult to obtain a good mixing condition in the wall layers. Besides, the mechanism of gas mixing was preliminarily discussed. Results indicate that gas mixing in the spout‐fluid bed is caused by both convection and diffusion. Diffusion other than molecular diffusion should not be neglected, especially at high spouting gas velocity or fluidizing gas flow rate. © 2005 American Institute of Chemical Engineers AIChE J, 2006