Fluidization of dissimilar materials is analysed theoretically in order to propose a model for the determination of the segregating behaviour of the two components upon suspension of the mixture. The attention is focussed mainly on systems in which diameter difference of the two solids contrasts with density difference, i.e. when the effect of size counteracts the effect of density. Starting from the case of a single sphere immersed in a bed of a different material, a force balance on the particle at bed suspension is shown to allow easily discriminating between conditions for floating or sinking of the test particle, depending solely on the properties of the test and bed particles and voidage. Then, moving to the force balance on one particle in a suspended two-component mixed bed, conditions for it to tend to float or sink are found, in this case involving also the effect of bed composition. The simple final form of the model, without adjustable parameters, allows predicting the initial segregation direction of the bed (i.e. defining the role of flotsam and jetsam components), as a function of size and density ratio, voidage and bed composition. The model results are shown to be qualitatively in agreement with the expected trends in terms of dependence of the segregation behaviour with the examined variables. Quantitative verification of the model is carried out with respect to the behaviour observed in a set of 37 systems taken from 12 different references. The model is currently limited to viscous flow regime and addresses only the initial segregation direction, while steady-state intensity is not in its scope. However, assuming full segregation, additional speculations on uncommon but theoretically possible segregation scenarios are discussed. (C) 2012 Elsevier B.V. All rights reserved.