A new jet fuel filtration monitoring system and technique is described which provides an assessment of the current performance and signals the end of the useful life of filter/separator and clay treatment elements. The need for such a system has long been recognized by the aircraft fueling industry to indicate when surface active agent type contaminants in jet fuels are no longer efficiently removed by clay treatment and/or interfere with the water coalescence function of the filter separator. The system, which has been developed and called a Filter Sidestream Sensor, operates on a sidestream with dynamic similarity to the main filter separator or clay vessel. It has been designed to provide a cumulative history of all factors which affect filter performance. Correlations between Filter Sidestream Sensor and main filter performance are provided together with data on the effects of surfactants–both natural and additive types, and the role played by water in coalescence deactivation.