The expansion of gas-fluidised beds has been examined in the range of Archimedes number from 100 to 10(5) using five grades of glass ballotini and two grades of lead shot of narrow size distribution, and Diakon particles of wide size distribution. A porous plastic distributor plate was used in beds of diameters 89 and 290 mm. Power law and exponential correlations of the expansion were examined. All of the experimental data were correlated within experimental error by the equation rho(b)/rho(mf) = exp[-0.0052U*(U/U-mf - 1)]. The dimensionless group U*, not apparently considered elsewhere, is defined as U-t(rho/mu g)(1/3) where U-t is the particle terminal velocity.