The drying kinetics corresponding to the falling rate period is commonly modeled using Fick's diffusion equation in fluidized beds. The appropriateness of the use of Fick's diffusion equation to represent the falling rate drying kinetics is the subject of the present paper. The estimated diffusion coefficient is normally the single kinetic parameter to assess the kinetics of the drying and is often used to compare the drying kinetics in different forms of drying. The dependence of the diffusion coefficient on the temperature and the concentration is well understood from the basic concepts of mass transfer; however, the diffusion coefficient estimated using Fick's diffusion model is found to vary with additional variables such as the solids holdup. The dependence of the diffusion coefficient on additional parameters is purely empirical and has not been justified in the earlier studies. Towards this, experiments are conducted in fluidization columns of varying diameter in order not to vary the quality of fluidization while varying the solids holdup on a larger scale. The estimated diffusion coefficient is found to vary by orders of magnitude with the variation in the column diameter ( solids holdup), necessitating the caution one needs to observe while comparing the kinetics of fluidized beds based on the diffusion coefficient. (c) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008