The relative enrichment of authigenic clay minerals with heavy rare earth elements (HREE) in Paleoproterozoic weathering profiles on the Baltic Shield is related to the presence of strong acids in drainage water. This is confirmed by the results of laboratory experiments on the treating granitoids by sulfuric acid solutions. Solutions obtained in the experiments exhibit the prevalence of potassium over sodium, whereas the concentration of potassium and sodium in initial granitoids is nearly the same.