A mathematical model was constructed and solved in order to describe quantitatively the dynamic behavior of the primary and secondary drying stages of the freeze-drying of pharmaceuticals in trays. The theoretical results were compared with the experimental data of the freeze-drying of skim milk, and the agreement between the experimental data and the theoretical results is good. Detailed model calculations have indicated that the contribution of the removal of bound (unfrozen) water to the total mass flux of the water removed during primary drying, is not significant. For this reason, it was found that one could not use effectively the experimental data of the primary drying stage to perform studies on parameter estimation and model discrimination for determining the functional form of the mechanism that could be used to describe the removal of bound water. The constitutive equation and the values of the parameters of the mechanism that could describe. for a given material of interest, the removal of bound water, should be determined by using the experimental data of the secondary drying stage. The results of this work indicate that one could neglect the mechanism of the removal of bound water in the mathematical model during primary drying, without introducing a significant error in the theoretical predictions. Two different mechanisms for the removal of bound water were examined, and it was found that a first-order rate desorption mechanism could describe the dynamic behavior of the removal of bound water satisfactorily. The model presented in this work has a very important practical advantage when compared with other models, because its expressions do not require detailed information about the structure of the porous matrix of the dried layer of the material being freeze-dried.