The two major metabolites of ibuprofen, 2-[4-(2-carboxypropyl)phenyl]propionic acid and 2-[4-(2-methyl-2-hydroxypropyl)phenyl]propionic acid, were labelled with tritium. The hydroxy-metabolite containing tritium at α- and β-carbons of the propionic acid moiety was prepared by catalytic reduction of the olefinic double bond in 2-[4-(2-methyl-2-hydroxypropyl)phenyl]propenoic acid with tritium-enriched