The mixing characteristic of large particles (Geldart BD type) has been investigated in a cylindrical gas-solid fluidized bed. Based on dimensional analysis, a correlation for the mixing index has been developed with the system parameters viz., average particle size of the mixture, initial static bed height, height of the particles' layer in the bed (from where the sample is drawn) and superficial velocity of the fluidizing medium. A theoretical model for the mixing index has been developed based on the counter flow of solids with circulation and horizontal dispersion. A comparison has been made for the values of the mixing index calculated by both the theoretical and the experimental models. (c) 2005 Elsevier B.V. All rights reserved.