A mathematical model describing the heat and moisture transfer within a solar drying system of rice was formulated. A numerical solution using MATLAB was implemented due to the many coupled PDEs and nonlinear algebraic equations. The model was checked for a range of the space steps and by comparison to analytical solutions for completed situations and was shown to contain no significant numerical errors. After estimating the best values and uncertainties of the system inputs the model was validated by comparison with experimental data for solar drying of rice. It was shown to be a very good mechanistic tool with advantages of simplicity and practical accuracy. The model accurately predicted the drying time and the temperature, and moisture content (MC) within the bed during drying except when a polystyrene drying pad was used. However, the model did not predict the experimental bed water activity (relative humidity) consistently well.