A mathematical model has been developed for continuous cross-dow dryers operating in diffusion-controlled regime. In the model, a cross-dow dryer is treated as a series of gas-solid contact stages, each purged by a stream of drying gas while wet solids are passing through. By analyzing the gas-solid interactions in each stage, boundary conditions for intra-particle diffusion were established and model equations obtained. The model correlates the amount of moisture removed to a number of process variables (diffusivity, partition coefficient, particle size, residence time, total number of stages, solids loading, drying gas humidity and purge rate) and allows effects of these variables on drying to be studied. For drying fine powders, the effect of reduced mass transfer efficiency due to particle agglomeration and gas channeling is taken into account in the model by factoring an efficiency parameter into the Fourier number. Model predictions of product moisture concentration based on laboratory-measured partition coefficient, diffusivity, and efficiency factor were found to closely match data from a commercial cross-dow HDPE powder dryer.