The onset of bubbling in a fluidised bed is defined in this study as the situation where the bubble rise velocity equals the maximum propagation velocity of voidage change. In this way the bed porosity at the onset of bubbling is correlated with the physical properties of the solid particles and of the fluidising fluid with the bubble size as critical parameter. Application of minimum fluidisation results in a scientific sustained description of the boundary between class A and class B fluid-solid systems. For ambient air, this A/B boundary is compared with the earlier introduced Ar=88.5 criterion and with the criteria proposed by Geldart and Molerus presuming a bubble size 5 times that of the representative particle. Data from literature related to the minimum bubbling superficial velocity are assessed to calculate the bubble size for real gas-solid systems. It appears that the bubble size for these literature data is about 1 to 5 times larger than the particle diameter except for a specific case. (c) 2005 Elsevier B.V. All rights reserved.