Two methods for the characterization of powder mixing (Geldart’s group A type powder) carried out in a fluidized column are compared and used for the determination of the mixing mechanism. The first one, well known, consists in taking samples from the fluidized bed after defluidization, and then analyzing the composition of the mixture. The second one is based on the follow-up of colored particles by means of optical fiber sensors. With this methodology the local concentration and the mixing time can be calculated. The radial and axial concentration profiles have also been determined and the gross circulation patterns identified. It appears that, for the chosen configuration, the mixing is conditioned by the movement of bubbles which provoke a circulation current involving a rising solids flow in the central zone of the column and a solids downflow near the walls.