A thin film dryer (TFD) device has been used to study the drying behavior of viscous products and their water diffusivities. The thin product film was dried convectively using different air velocities, air temperatures, air humidities, initial solid concentrations, initial film thicknesses and optional addition of agar. A mathematical model was developed based on the external and internal heat and mass transfer with ideal shrinkage. A coordinate system attached to the solids was used and later transformed to the initial height to facilitate the numerical implementation. The diffusion coefficient of the product was expressed as a function of water content and temperature by fitting the model to the experimental drying kinetics (for different conditions) obtained for a given product. The diffusion coefficients obtained for maltodextrin and PVP show good agreement with literature. They are significantly reduced when the material changes from the rubbery to the glassy state and can be well expressed by the Williams-Landel-Ferry (WLF) equation,