Abstract： This paper concentrates on the principles and measurement of interfacial tension and its value in predicting the contamination of oil e.g. by organic polar compounds which are formed by the interaction of oil, water and oxygen and reduce the functionality of the oil. These contaminants are attracted to the oil/water interface and as a result significantly lower the interfacial tension. A pure interface is compared to one modified by surface active additives for the effect on interfacial tension, various methods for measuring interfacial tension are described and a method using interfacial tension to determine the polar and dispersed components of a liquid will be explained.. The ASTM Standard D971 will be discussed and results shown for both transformer and lubrication oils. The question The Surface Tension Test - Is it Worth resurrecting?1 can be discussed during Question Time.