The mechanism underlying mixing/segregation of binary particles in liquid fluidized beds is reviewed and investigated in this paper. Binary mixtures of particles, when fluidized, sometimes segregate and display a behaviour called layer inversion. At low fluid velocities, one of the components is primarily found in a discrete layer at the bottom of the bed, while the other is predominantly at the top. At higher fluid velocities, the order of arrangement is reversed. The literature provides a variety of explanations for this phenomenon, derived from quite different theoretical bases. A comparative analysis of these different approaches is presented here together with the experimental results available in the literature. Based on the best model, further experimental investigation is carried out to provide; (i) comprehensive criteria to predict whether a given binary mixture of any type (both size and density variant, size variant only, density variant only) will mix/segregate or show layer inversion, and (ii) mixing/segregation regime map in terms of size ratio and density ratio of the particles for a given fluidizing medium. Therefore, knowing the properties of given particles, a second type of particles can be chosen in order to avoid or to promote segregation according to the particular process requirement.