This paper analyses the process of fluidization of mixtures of two spherical solids differing only in density and addresses the relationship between bed suspension and component segregation. It is shown that relating the progress of segregation to the excess gas velocity mu-mu(mf), a parameter that should represent the intensity of bubbling, gives way to serious errors, due to the fact that extending to binary beds the concept of minimum fluidization velocity leads to overlook some important aspects of their behaviour. clearer description of the fluidization properties of density-segregating mixtures is that based on the definition of the 'initial' and 'final fluidization velocity' of the binary bed. Such an alternative approach provides an effective representation of the actual behaviour of these systems, as it correctly accounts for all the variables which affect their phenomenology of fluidization. (C) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.