Many factors affect the rate of deterioration of a fat used for deep fat frying. Because of the complexity of the problem, there is no single procedure which will yield reliable results in all situations. Determination of total polar materials in a frying fat provides the most reliable measure of the extent of deterioration in most cases. For monitoring a frying operation in which there is no dilution of the frying fat by the fat in the food being fried, the change in the dielectric constant is the simplest of the methods judged reliable. Free fatty acid determinations by titration and peroxide value are not recommended procedures for measuring frying fat deterioration.