Abstract: A method and apparatus for determining the presence of surfactants in a liquid, such as a jet fuel or diesel fuel, by using the capillary rise technique, wherein an enclosed evacuated space is provided which is partially filled with water. A capillary tube is provided in the enclosed evacuated space having one of its ends submerged in the water. The liquid whose interfacial tension is to be measured is introduced into the evacuated space and the amount of rise in the capillary tube is measured. From the amount of rise in the capillary tube, the interfacial tension can be calculated. A high interfacial tension generally indicates the absence of surfactants which can affect the operation of coalescing filters which are designed to remove water from jet or diesel fuel before use.