Experimental work on the entrainment of an FCC powder was carried out in 76 mm and 152 mm glass columns. Fluidization tests were carried out batchwise and continuously at velocities between 0.2 m/s and 0.8 m/s. Non-cohesive fine powders and cohesive superfine powders were added to a coarse base FCC powder in proportions which ensured that the mixture remained within Geldart＇s Group A. The entrainment rate constant, K-i infinity*, is found to depend slightly on the fines concentration in the bed. Our work has confirmed that there is a critical particle size at which K-i infinity* no longer increases as particle size decreases, and may even decrease. Several mechanisms are postulated to explain this but from the experimental results it is concluded that interparticle adhesion forces between the very fine particles play an important role. An empirical correlation to predict K-i infinity*, below the point where levelling off occur is proposed.