Summary (1) It has been shown that ascorbic acid plays an important part in the oxidative deterioration of milk fat at the end of its storage life, as determined by the re-emulsification test, resulting in the development of objectionable flavors and losses in vitamins A and E and the carotene content of the fat. The susceptibility of fat to this type of deterioration is determined primarily by the treatment of milk, the temperature of pasteurization, the type of product, the conditions of storage, and to a lesser extent upon the direct and immediate effect of the exposure to light. (2) The exposure of pure fat to light generated by mercury vapor lamp (1400 foot-candles) slightly affects its vitamin E content. However, it lowers the resistance of vitamin E in the stable fat to deterioration as determined by the re-emulsification test. (3) The re-emulsification test was found to be useful in determining the end of the storage life of fat when the fat was obtained from products which have not as yet shown any apparent changes in their flavors. This view is supported by the observations showing that the depletion of cream of its total vitamin C content, either by oxygenation, or by hydrogen peroxide, has prevented the development of the objectionable flavors for 12 and 24 months at −17.7 to −16.1° C., respectively. In the reemulsification test, however, the fat obtained from oxygenated milk pasteurized up to 76.6° C. lost its ability to resist the foregoing type of deterioration at the end of 4 to 6 months of storage, depending upon the conditions of processing. (4) Only the fat from butter churned from cream pasteurized at 71.1 and 76.6° C. and the pure fat retained their abilities to resist deterioration in the re-emulsification test at the end of two years storage at −17.7 to −16.1° C.