Authigenic micrite from a playa in the northern Sahara has been dated by both the U/Th and 14 C methods. The U/Th ages can be a few thousand years greater or less than the 14 C ages obtained on the smallest crystals of micrite. The latter are considered to form a reasonable chronology for the Holocene deposits. The deviations of the U/Th ages are explained by quantifiable losses and gains of uranium under changing redox conditions. Under conditions where U is conservative (reduced sediments with low permeability), the U/Th method can provide good chronologies for lacustrine deposits.