AbstractA comparison was made of the equilibrium mixing/segregation behavior of binary mixtures of different‐sized, equal‐density glass beads in two‐ and three‐dimensional fluidized beds. Several empirical equations for mixing index were examined, all involving a dimensionless function of excess gas velocity. This function, which appeared to largely account for the effect of aspect ratio, was found to optimally correlate the data for both types of beds. However, a much better fit was obtained for the threedimensional bed data, most probably because wall effects significantly retarded mixing in the two‐dimens