Piecemeal derivations of a model in the literature for predicting the degree of particle segregation by size of a binary particle mixture in a liquid-solid fluidized bed are unified and simplified. It is shown that the degree of particle segregation at a given liquid velocity can be represented quantitatively by the reduced bulk density difference of the two particle species each fluidized separately at the same liquid velocity and that for equal-density particles the reduced bulk density difference is given by the corresponding voidage difference of the two species. This criterion indicates an increase of particle segregation as the particle size ratio (large/small) increases, as the fluid velocity and, hence, the voidage level increase, and as the Archimedes number Ar decreases from the Newton regime (Ar > 85 000) to the Stokes regime (Ar < 4).