Fluidized bed coating with supercritical carbon dioxide (SC-CO2) is a promising technology for processing microparticles of high added value. Up to now, mainly particles with sizes higher than 100 mu m have been used. The main challenge is to be able to uniformly coat particles with smaller sizes. The key point is to get a good mixing of both, solids and Supereritical fluid. The quality of the liquid mixing has been analyzed by means of residence time distribution studies of fluidized Geldart type A glass beads (d(p,s) = 65 mu m; d(p,s) = 176 mu m) at pressures from 8 to 14MPa and temperatures from 35 up to 50 degrees C. Experimental results have been correlated satisfactorily with a one-parameter model (N tanks in series) and show that the mixing is acceptable with two or three tanks in series, with the exception of experiments at fluid velocities below twice the minimum fluidization velocity and small initial volume of solids where the mixing is worse. (C) 2007 Elsevier B.V. All rights reserved.