The stability of reduced and total ascorbic acid in a dehydrated model food system was determined as a function of water activity, moisture content, oxygen and storage temperature. Sorption isotherm data obtained at 10, 20, 30 and 37°C for the model system were used to calculate the BET monomolecular moisture content at these respective temperatures. The model system containing 11.25 mg of ascorbic acid per 100g (25% RDA) was equilibrated at water activities below, at and above the water activity corresponding to the calculated monomolecular moisture content for the adsorption isotherms The samples were then sealed in tin cans and stored under isothermal conditions to prevent any shift in water activity during storage. Reduced, dehydro- and total ascorbic acid levels were measured using an automated o-phenyl-enediamine fluorometric assay procedure and their rates of degradation determined for each storage temperature and water activity. Under all storage conditions total and reduced ascorbic acid losses could be described by first-order kinetic functions. Rate of ascorbic acid destruction was dependent upon water activity, moisture content and storage temperature when stored in containers with no headspace. The rate of ascorbic acid destruction was shown to dramatically increase with the presence of oxygen.