Oxygen availability in different media during heat treatment (8 h at 80°C) and the related vitamin C loss was assessed. Dissolved oxygen in water containing 3 mmol kg-1 of ascorbic acid decreased initially and seemed to be replaced by oxygen from the headspace in the course of time, as oxygen values increased again. In apple puree and carrot puree in contrast, oxygen was depleted within 60 min. Vitamin C in ultrapure water was stable even in the presence of oxygen. A trigger seemed to be crucial to launch vitamin C degradation. Fe3+ ions added to water, but also the media Mc Ilvaine citrate-phosphate buffer (pH 3.5) or apple puree, initiated degradation. Adding Fe3+ ions to apple puree did not accelerate vitamin C degradation but shifted the equilibrium between ascorbic acid and dehydroascorbic acid to the latter. Oxygen deprivation stabilized completely vitamin C, independently of the medium tested. A temperature decrease to 70 or 60°C, in contrast, had no effect on the degradation extent of vitamin C in water containing 20 mmol kg-1 Fe3+ ions but led to complete stability in apple puree.