The 4-quinolones having the urea or carbamate moiety at the 2-position showed the facile deuterium-hydrogen (D-H) exchange of the 3-H proton in deuteriotrifluoroacetic acid-deuteriodimethyl sulfoxide (3:1) at 60 degrees C, whereas the 4-quinolones possessing the carboxylate or carbohydrazide group at the 2-position and 2-substituted 4-methoxyquinolines represented no D-H exchange of the 3-H proton under the same condition. The aforementioned D-H exchange was found to require both the tautomerization of the 4-quinolone into 4-hydroxyquinoline in strongly acidic media and the nitrogen functional group at the 2-position.