Abstract: In a method of evaluating the thermal stability of hydrocarbon liquid fuels, the fuel flows over a heated tube that causes the fuel to decompose and deposit a residue as a film on the surface of the heated tube. The thickness of that residue film is a measure of the fuel's thermal stability. To measure the film's thickness, the film is illuminated with light that is substantially monochromatic. The light reflected from the surface of the heater tube interferes with the light reflected from the overlying surface of the residue film on that tube and thereby causes variations in the intensity of the reflected light. Those variations in intensity of the reflected light are detected to obtain a measure of the thickness of the film. In one embodiment of the invention, the heated tube is situated in an enclosed housing through which the fuel is caused to flow and the thickness measurement is made while the film builds up on the heated tube.