Positrons have numerously proven their enormous sensitivity for the non-destructive detection of defects. By experiments performed at NEPOMUC it could be shown that also metallic agglomerations can be characterized with highest sensitivity. Not only the detection of 0.1nm tin embedded under 200nm of aluminium was reported but also a strong size effect of the positron trapping at nm-sized gold clusters was discovered. The non-desctructiveness of positrons enables in-situ investigations in the volume of a metallic sample, where the generation or solution of nano-clusters can be investigated. By using monoenergetic positrons, the depth of the investigated volume can match an embedded layer (see figure) and the high brightness of the NEPOMUC beam together with a novel detectorsystem and evaluation method allows the unrivalled short measurement time of one hour per CDB spectrum.