The floating gas-solid fluidized bed (FGSFB) is a large-particle fluidized bed confined in a tapered column with inserted grids. By application of two types of particles in an FGSFB, a wide variety of operations can be carried out. The visual appearance and the hydrodynamic behaviour of such binary FGSFBs is reported in this paper. Two extreme types of behaviour were observed in the experimental set-up: (1) almost complete separation occurs in beds of two types of particle with the same diameter but a particle density ratio ρp, 2/ρp, 1 of ca. 2 (±1400 vs ±700 kg/m3); (2) a relatively well-mixed situation is found in beds with two types of particle having the same density but a particle diameter ratio dp, 2dp, 1 of ca. 2 (3.0 vs 1.7 mm). Experimentally determined pressure gradient values were always within the range of those measured for FGSFBs of the single-particle type. A measure for the separating force, the so-called separation tendency W, is derived and shown to be quantified by single-particle properties. Its value can be used to predict whether complete or intermediate mixing occurs, or if the binary FGSFB will be completely separated.