Binary systems of particles of different size but equal density are fluidized in a 30-cm diameter bed with a perforated plate distributor. This work describes the extensive experimentation, and relates the mixing/segregation properties to the visible bubble flow rate, the particle size ratio, and other parameters of minor influence. Experimental data are expressed as mixing index, correlated in terms of the decisive parameters. Comparison with previous empirical equations for the mixing index is also included. The excess gas flow rate required to avoid segregation in a fluidized bed of wide size distribution powders can be calculated from the expression for the mixing index (Eq. (15)).