Various industries use fluid bed dryers for the drying of larger granules produced by extrusion, pan agglomeration or spray granulation. During such drying processes secondary agglomeration is undesired due to an increasing amount of oversize particles. In the current study the drying of granules containing amorphous substances is modelled. The model is based on the laws of heat and mass transfer as well as heat and mass balances around a differential volume element of the fluid bed. For each location in the bed and for different drying parameters the calculated moisture and temperature values are used to estimate the surface viscosity of the granules. The calculated viscosity allows estimating the risk of secondary agglomeration of granules. Viscosity values which are estimated for different drying conditions are compared with the experimentally determined amount of oversize particles. An increasing amount of oversize particles can be observed if the viscosity of the amorphous substance is below 10(4) Pa s. (C) 2010 Elsevier B.V. All rights reserved.