A theory is developed for the kinetics of moisture migration in packaged multi-component food mixtures, such as cereal-fruit mixtures, mixed candy or freeze dried instant meals. The theory assumes the individual food components to be characterised by their initial water activities, isotherm slopes and a function describing the time course of moisture uptake under constant humidity conditions. the step-response functions. When specifying the product properties compositions and net weight and package parameters such as area and water vapour permeability, the time development of internal moisture redistribution and water uptake into the package can be calculated. Calculations on realistic packages containing a cereal-raisin mixture show that the headspace water activity attains the water activity of the cereal flakes within one minute. On a timescale of one year the processes of moisture transfer from raisins to cereals and permeations through the package film take place. For the raisin-cereal mixtures under consideration the product moisture transfer from raisin to cereals limits quality. Shelf life calculations based on the kinetic theory show that product composition and package permeability optimal for the overall product quality can be identified. The calculations demonstrate that information from focused experiments on individual food components can be used to predict storage stability on a number of mixtures and under different conditions. (C) 2003 Elsevier Science Ltd. All rights reserved.