Abstract Steady-state tracer-gas measurements were carried out in a circulating fluidised-bed (CFB) boiler and in a cold experimental (CFB) unit. The main focus was on the radial spread of gas in the core region of the transport zone under different operating conditions. Helium (He) was used as tracer gas, and the local He-concentration was determined by a mass spectrometer. The fluidisation velocity ranged from 1.2 to 4.3m/s in the boiler and from 1 to 6m/s in the cold unit. The bed material was silica sand with an average particle diameter of 0.32mm in both units. Measurements were also carried out in the bottom bed and splash zone of the boiler for comparison with those of the transport zone. In the cold experimental unit, a greater range of operating conditions could be covered than what was possible in the boiler, including a case without solids. In the core of the transport region of the boiler, there is no backmixing of gas and the horizontal mixing of injected tracer gas is characterised by an axi-symmetric spread of tracer gas around the centre line of injection. The horizontal gas-mixing rate is quantified by a dispersion coefficient, which is above 0.01m2/s in the boiler and around 0.005m2/s in the cold unit. The difference in dispersion coefficients of the boiler and the cold unit is due to the difference in size. The present results were compared with literature data of dispersion obtained with and without solids in CFB risers.