Lead isotope composition of the secondary uranium minerals kasolite and wölsendorfite has been examined in a masspectrometer. The material originates from uranium mineralizations, radioactive pegmatites and a site with no known primary uranium mineral source. The lead of the secondaries consists of varying proportions of common lead (rock lead) and radiogenic lead formed by decay of uranium and thorium. Uranogenic lead dominates in all but one of the samples examined. The mineralization age is approximated by examining secondary uranium minerals. The secondaries are usually depleted in uranium in relation to lead, but the reverse relation also exists. The examined minerals, with one possible exception, seem to have been formed within recent geological time.