An apparatus and method for determining the thermal stability of fluids, particularly for testing the deposition characteristics of fuels, comprising a conduit system which connects in series; a test fluid reservoir having a movable piston capable of discharging the fluid, a heater section in which is mounted a heated tubular member, a time reactor of a sufficient volume to increase the residence time of the fluid therein, and a water displacement reservoir. The system also incorporates a filter located at an outlet of the heater section designed to trap deposits which may form due to the heating, and a similar filter located at an outlet of the time reactor designed to trap further deposits that may form with additional time. A differential pressure measuring device is connected across each of these filters to measure the pressure drop therethrough. The apparatus also contains a water balanced reservoir and pressure regulator whereby the entire system may be maintained at a constant pressure and prevent the discharge of volatile fluids to the atmosphere.