This paper reviews the importance of temperature gradient induced moisture diffusion in powder beds. Redistribution of moisture by this mechanism is important for a wide variety of products, especially those which are crystalline in nature. Moisture transfer in these products can result in very high local water activity and lead to microbial spoilage and caking problems. A conceptual model was developed for heat and moisture transfer due to the application of thermal gradients across a one-dimensional powder bed. The validity of the key assumptions in the model formulation were demonstrated and a mathematical model was derived and solved numerically. The model was validated against experimental data collected using crystalline lactose powders as a working example. (c) 2008 Elsevier Ltd. All rights reserved.