Abstract: A thermal oxidation tester is shown for determining thermal stability of a fluid, particularly hydrocarbons when subjected to elevated temperatures. The tendency of the heated fluid to oxidize and (1) form deposits on a surface of a heater tube and (2) form solids therein, are both measured at a given flow rate, temperature and time. The measured results are used to determine whether a fluid sample passes or fails the test. At the beginning of each test, the test sample is aerated with dry air until saturation. Simultaneously, the test equipment is primed to remove pockets of air there from. After each test, the test equipment is flushed to remove the test sample there from.