Studies on freeze-dried model systems containing methyl linoleate and various additives have shown that water content plays an important role in controlling the activity of antioxidants and catalysts; water can counteract the catalytic activity of certain metals, and can enhance the antioxidant effect of some chelating agents. Different additives, however, respond differently to the effects of humidification, especially at high water activities. Data are presented which show that complexes of managanese and histidine show enhanced prooxidant activity at high water contents, while those of cobalt and histidine become more antioxidant. The effectiveness of EDTA as a chelating agent is also enhanced as the moisture content is increased. However, the effects of humidification are dependent on other components of the system, including the nature of the hydrophilic support.