The hold-up of particles has been studied in two gas fluidised beds of 89 and 290 mm diameter. Particles of different materials, density from 1228 to 11400 kg/m(3) and size from 50 mum to 1 mm were fluidised. Four distributors were examined in the larger diameter bed. Power law and exponential correlation of the expansion were examined. All of the experimental data for open beds and for bed with tubular inserts with different distributors were correlated within experimental error by the equation, rho(b)/rho(mf) = exp [-lambda U*(U/U-mf-1)]. The dimensionless group U* is defined as U-t(rho/mug)(1/3) where U-t is the particle terminal velocity. The constant lambda has a value of 0.0052 for open beds and 0.0036 for fluidised bed with tubular inserts.