The effective moisture transfer diffusion coefficient (D-eff) of ready-to-eat breakfast cereals (RTE) is determined either from art individual particle or a bulk of material in the Literature. The D-eff is assumed to be the same in both cases which can be dependent on the material thickness. In this study, the effect of bed depth of a bulk Of two types of RTE cereal on D-eff was investigated at several bed depths at 75% RH. Both linear and nonlinear solutions of Fick＇s 2nd Law were applied to determine D-eff from the weight vs time data. The magnitude of D-eff increased with an increase in bed depth. Since the captured air between the particles in the bulk volume contributed significantly to the moisture transfer in the system, the overall D-eff of the system increased as the total volume of the captured air increased at higher bed depths. The relation was shown to have a square of the thickness dependence.