Vitamin C occurs in relatively high concentrations in fresh and processed fruits and vegetables but is found to a lesser extent in animal tissues and animal-derived products. Nearly 90 % of vitamin C in the human diet is obtained from fruits and vegetables but this can be indirect by way of commercially prepared fruit juices. These juices are often enriched with vitamin C which has been synthetically prepared. There is a wide range of such juices on the New Zealand market, and they are a significant source of dietary vitamin C for many in the population. The focus of this research is on the Keri range of juice products.The present study monitors the fate of vitamin C during storage of Keri juices up to the best-before date, and under a range of other storage and consumption situations. Two methods were adopted for determining ascorbic acid (AA, the chemical identity of vitamin C). These were the titrimetric method, which is based upon the reduction of the dye 2,6-dichlorophenolindophenol by AA in acidic solution, and liquid chromatography, which is used to separate AA from its immediate oxidation product dehydroascorbic acid. In the latter method these two analytes can be measured independently. The liquid chromatography was less successful than the simpler titrimetric method, so most of the work was done by titration. However, the concentration of dehydroascorbic acid, which has vitamin C activity in vivo, remained uncertain. Moreover, the titrimetric method could not be applied to juices with high purple anthocyanin concentrations, like blackcurrant, because the colour change at the titration end point could not be detected. pH adjustment to change colour was ineffective, and decolourisation with charcoal led to the rapid and complete destruction of AA. The concentration of AA in Keri juices at the time of manufacture were always much higher than claimed on the labels. Storage for up to nine months at room temperature resulted in a loss in AA of between 37 and 68 %, depending on the juice and exposure to fluorescent light. However, the time of storage was a much more dominant factor than light exposure. The kinetics of loss, straight lines, were most easily explained by an aerobic model of AA degradation from oxygen diffusing across the polyethylene tetraphthalate bottle wall. Overall, the label claims made were defensible in terms of the best-before date, because it took at least 100 days of storage before the AA concentration in the most susceptible juices fell below the claimed value. This is because these drinks are fast moving consumer goods and storage beyond 100 days is unlikely. (Nonetheless, the supplier (Keri Juice Company) has since adopted its new unitised method of formulating juice. This has resulted in an initially higher concentration of vitamin C as compared to the juices under investigation.) In the nine months storage experiment there was some evidence for the presence of dehydroascorbic acid in blackcurrant drinks, but not in another three juices. Pasteurisation during preparation of these drinks resulted in up to 7 % loss of AA, probably due to oxygen dissolved in water, and accelerated by heat of pasteurisation. Higher temperatures in later storage also accelerated losses. Progressive exposure of juice to air during simulated consumption of 3 L bottles over a week also accelerated losses. Finally, exposure to sunlight in a diurnal temperature environment accelerated losses five-fold higher than in total darkness. Filtration of ultraviolet light approximately halved the loss due to sunlight. Overall however, it can be concluded that AA in the Keri range of juices is very resistant to degradation of AA.