Shelf life is an important aspect of product quality. Extensive accelerated aging testing is carried out to ensure that cigarettes maintain their moisture levels within acceptable limits for as long as possible. Such testing, however, does not in general lead to a detailed understanding of the mechanisms involved in moisture transfer through cigarette packs. To that end a simple predictive mathematical model has been developed. The rate of moisture loss or gain is shown to be given by the product of a pack rate constant and the water vapor pressure differential across the pack. The value of the pack rate constant is determined by the permeability of the overwrap and pack seal quality. The water vapor pressure inside the pack is a function of the moisture content of the tobacco and its sorption isotherm. If the temperature is assumed to be constant, the rate equation yields simple analytical solutions. The resulting kinetic equations were found to be in excellent agreement with experimental moisture loss data.