Abstract The effective moisture transfer diffusion coefficient ( D eff ) of ready-to-eat breakfast cereals (RTE) is determined either from an 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.