Some blister packages are designed to protect pharmaceutical drugs from environmental factors such as moisture. Moisture uptake by anhydrous solids stored in these packages is primarily controlled by adsorption/absorption phenomena and can be predicted if the thermodynamic and transport properties of the package constituents (esp., the drug and the polymer barrier phases) are known. In this thesis, a quantitative metric for the shelf life of a blister package is developed in terms of parameters associated with individual components of the package and the environmental conditions associated with storage. The adsorption/absorption model developed herein assumes that the moisture distribution rapidly attains a pseudo-steady state profile within the polymer barrier phase and within the air gap phase. This pseudo-steady state approximation (PSSA) is justified based on an exact unsteady-state analysis of a conjugate absorption problem with linear isotherms. A commercial finite element code (COMSOL MULTIPHYSICS 3.3) was used to analyze the non-linear boundary value problem resulting from the use of a GAB-adsorption isotherm at the solid-product/air-gap interface. The resulting mathematical model is consistent with an earlier comprehensive experimental study of moisture uptake by blister packages containing 20 mg Deltasone^® tablets (see Allen, 1994). The PSSA model provides a practical tool for estimating the shelf life of blister packages and for evaluating testing protocols.