An experimental verification of the scaling relationships for bubbling gas-fluidised beds has been carried out by measuring particle cycle frequency. The Positron Emission Particle Tracking technique was used, which enabled non-intrusive measurements of solids motion in three dimensions with a spatial resolution of similar to2 mm under the conditions of this work. Three cylindrical beds (70, 141 and 240 mm ID) equipped with multiple orifice-type distributors were tested. It was shown that for geometrically similar beds, the pair of Froude numbers based on the minimum fluidisation and excess gas velocities was sufficient for similarity at the viscous limit. The experimental results showed little influence of the gas-to-particle density ratio, but a small effect of particle-to-bed diameter ratio when the change in the ratio was significant. The experimental results also indicated a possible relaxation of the viscous limit up to a particle Reynolds number of about 100. More experimental studies are needed to confirm the validity of such a relaxation.