The mixing and hydrodynamic behaviour of binary fluidized beds of particles of same size and differing density has been investigated experimentally by using radiation based non-invasive techniques, viz, radioactive particle tracking (RPT) and dual-source densitometry. The effect of air inlet velocity and bed composition has been investigated. RPT experiments are performed individually for both the solids by tracking one particle at a time. Many interesting findings are discussed, for example, such as the heavier (jetsam) phase is seen to be fluidized by action of the lighter (flotsam) phase even below its minimum fluidization velocity. Further, it has been found that while with increase in air inlet velocity, the extent of mixing of solid phases increases; even at very high air velocities the jetsam phase is not completely mixed. The important role played by the particle-particle collisions in the fluidization of binary mixtures is quantitatively highlighted.