The Water Separation Index, Modified, has been accepted as the standard procedure for assessing the water separating characteristics of a fuel. The significance of this test in relation to filter/separator performance and life, however, has not been well established. An evaluation of the WSIM test has been made in an attempt to establish this significance. This evaluation has shown that the test is sensitive to the presence of surface active materials in a fuel. The response to such materials is reasonably quantitative, although all materials do not respond in the same manner. Filtration tests confirm the sensitivity and indicate that a WSIM rating of less than 100, possibly as low as 75, can be considered acceptable.