Based on a simplified approach to turbulence modelling, a qualitative equation is derived for the turbulent mixing of gas in circulating fluidised beds. As a result, an explicit equation for the local turbulent diffusion constant in two-phase flows is presented. The model takes into account operating parameters such as particle diameter, gas viscosity and shaft diameter. Predictions are shown to agree well with experimental data on gas mixing in a cold rig with a cross-section 0.3 × O.4 m. They also compare well with literature data on local turbulence intensities in a 30 mm tube measured with LDA-technique.