[en] Retinyl acetate (4a) has been prepared with a tritium label at the C-11 and C-12 positions by partial reduction of oxenin (1) with tritium gas followed by acetylation and rearrangement. Specific activities of up to 40 curies/mmole have been attained. By alkaline hydrolysis, retinol (5) has been obtained and derivatized as one of several retinyl esters or has been oxidized to all-trans retinoic acid (6) having equally high specific tritium activities. From beta ionone (12), 13-cis retinoic acid (10) has been elaborated in a variety of isotopically labeled forms. A key reaction in the sequence, the Wittig coupling of the triphenylphosphonium derivative of vinyl beta ionol (15a-c) and the butenolide, 5-hydroxy-4-methyl-2(5H)-furanone (8), provides access to 13-cis retinoic acid and its 4-oxo metabolite labeled with either isotopic hydrogen or carbon. (author)