Gas containing tracer was injected continuously at a point near the distributor into fluidised beds, diameters 2 in. and 6 in. respectively, maintained by tracer-free air-flow at or slightly above incipient fluidisation rate. A stream of bubbles containing tracer gas was produced. In the 2-in.-diam. bed the dense phase gas composition was radially uniform but the concentration increased with height presumably due to gas exchange with tracer-rich bubbles. In the 6-in.-diam. bed, radial variations in composition of the dense phase were apparent. However, the average concentration over a cross-section increased with height also. Exchange coefficients were approximately determined and fair agreement was obtained with the values suggested by Davidson and Zenz. The solids employed were glass spheres from 50-380 μ diam.