This paper describes the development of a liquid flow reactor which is capable of emulating the JFTOT apparatus conditions but which is constructed of simple off-the-shelf components. This new gravimetric fuel thermal oxidation tester allows the determination of deposit weights directly, simply and non-destructively by gravimetry. This new device is quite compact and offers vastly improved flexibility of operating conditions including a wide range of test temperatures, pressures, flows and reactant types. It is ideally constructed for the attachment of ancillary analytical devices. The results of tests with this device on 15 specification jet fuels at standard ASTM D3241 (Jet Fuel Thermal Oxidation Tester Procedure) conditions are presented. The gravimetric tester yielded quantitative and repeatable measurements of both heater strip deposits and fuel insoluble concentrations.