A series of Caledonian igneous rocks from East Greenland have been dated by means of UPb isotope determinations on zircons. All discordias obtained give too low intercept ages, in one case more than 200 Ma younger than the RbSr mineral isochron age from the same sample. Leach experiments show that recent or subrecent loss of lead is responsible for the geologically meaningless ages obtained. Further it is shown that the recent lead loss is not caused by laboratory treatment, but must be related to natural processes. X-ray investigations combined with transmission electron microscope analyses show a correlation between recent lead loss and the chemistry of the investigated zircons.