Abstract: A method for detecting contamination of a liquid, particularly a Quench Oil, comprises introducing a sensing element at a high temperature into a liquid, allowing the sensing element to commence cooling, and thereafter timing the cooling of the sensing element between two selected temperatures. The time thus measured is compared with a pre-set time, to provide an indication of contamination of the liquid. In apparatus for carrying out the method, a slug of stainless steel in which is embedded a thermo couple is heated in a furnace and then lowered into a receptacle containing the liquid under test. These operations, and the timing and comparison, are effected in a pre-determined automatic cycle.