In a liquid fluidized bed consisting of binary solid particles differing in size and density, the mixing of the two kinds of particles occurs and the degree of mixing is controlled by an equilibrium force balance relationship. A model to predict the volume fractions of particles in the mixed layer is developed in this paper on the basis of the force balance relationship of each particle species. In describing the drag forces of particle components 1 and 2 in a mixed layer, two hypothetical mono-component beds, bed (1*) and bed (2*), are introduced. The bed (i*) (i = 1, 2) consists of only one particle component i, and it is assumed that the drag force acting on the particle component i in this bed is equal to that acting on component i in the mixed layer. This assumption, coupled with the Richardson and Zaki formula, leads to a modified Richardson and Zaki formula for computing the composition of the mixed layer in a binary solid-liquid fluidized bed. The validity of this model is tested against experimental data.