To get information on extremely small organic deposits in ultra high vacuum systems the sensitivity of most of the conventional methods for thickness measurements is not high enough. On the other hand the radioactive tracer method has shown its high sensitivity and wide versatility in many fields. Tritium with a half life of 12.3 γ and a mean β-energy of 5.4 keV was choosen as tracer isotope. A method is described for Tritium-labelling diffusion pump oils with specific activities up to 100 mC/g. Using the liquid scintillation counting technique one can detect deposits down to below 1010 molecules/cm2. First results with this Tritium labelled pump fluid are given under different operating conditions in an all metal ultra high vacuum system.
