A new laboratory scale coalescence test, designated the Exxon Coalescence Test or ECT, was developed which allows the water coalescence of aviation fuels to be investigated under controlled conditions. The ECT is essentially a small-scale filter/separator incorporating both a coalescer and separator stages and operating at a 800-fold reduction in flow rate from a full-scale filter/coalescer element. The effect of a number of commercial fuel additives, antioxidants, corrosion inhibitors, anti-icing additives, and static dissipator additives, on water coalescence in JP-5 aviation fuel were determined using the ECT. Single Element Tests, conducted using full-scale military filter/coalescer elements, validated the results from the ECT. The ECT promises to be an effective tool for further investigation of the factors affecting water coalescence in filter/separators.