Horizontal gas dispersion is studied by tracer-gas measurements and smoke visualisation in the riser of a 1/9-scale model of the Chalmers' circulating fluidised-bed (CFB) boiler, with complementary measurements in the boiler. The model was designed and operated according to fluid dynamic scaling-laws. The results are based on the time-average horizontal dispersion of gas, dynamics of the measured concentration of tracer gas and smoke visualisation of the gas flow. The fluidisation velocity in the boiler ranged from 1.2 to 4.3 m/s and the bed material was silica sand with an average particle diameter of 0.3 mm. When the cold scale-model was operated to resemble the conditions of the tracer-gas measurements in the full-scale boiler, the results from the two units were similar (scale model operated with a fluidisation velocity of 0.4 to 1.4 m/s and with 0.06-mm bronze particles). The scale model also covers conditions that cannot be reached in a full-scale boiler. For instance, changes in the characteristics of the gas mixing during a gradual increase of particle concentration in the riser, starting from an empty riser, could be studied. The gas flow is characterised by a combination of small-scale motion (widening of a gas plume) and large-scale motion (a meandering motion of the whole plume), Both types of motion contribute to the mixing of gas. The large-scale motion in the transport zone is linked to the bubble eruptions in the bottom bed and the horizontal dispersion is higher in the bottom- and splash zones than in the transport zone. The dispersion rate is higher at low gas velocities than at high velocities because of a great contribution from the large-scale motion generated by bubbles at low particle circulation rates.