Excited-state processes in 6-hydroxyquinoline (6-HQ) were investigated in acidic, basic, and neutral media. When 6-HQ is in the quinolinium form (i.e. with protonated ring nitrogen) in acidic aqueous solutions, the hydroxyl group behaves like a very strong acid in the excited state : deprotonation occurs even in a solution of 10 M HClO4! Such a very high photoacidity is explained by the absence of proton back-recombination, as shown by time-resolved measurements, rather than by a high rate constant for deprotonation. The lack of proton recombination, surprising at first sight in very acidic solutions, is shown to be due to intramolecular electron transfer from the hydroxylate group to the positively charged pyridinium ring as soon as the proton is ejected : this leads to an excited tautomer predominantly in a quinonoid form. Deexcitation of this tautomer occurs via reverse electron transfer, the ground-state form being predominantly zwitterionic; this route of deexcitation is mainly nonradiative as in the case of betaines. In fact, solvatochromism experiments performed on the parent compound 1-methyl-6-oxyquinolinium confirm the strong analogy with betaines and in particular with betaine 30 which is known to undergo nonradiative deexcitation, via ultrafast intramolecular electron transfer, toward the ground-state zwitterionic form. When 6-HQ is in the phenolate form in basic aqueous solutions, the heterocyclic nitrogen atom behaves as a very strong base in the excited state, and proton uptake is coupled to fast intramolecular electron transfer from the hydroxylate group to the adjacent ring. Finally; the photophysical behavior of 6-HQ in the neutral form, and in particular its very low quantum yield, can be interpreted in terms of double proton transfer coupled to intramolecular electron transfer. Therefore, it is concluded that all the excited-state processes in 6-HQ can be explained along the same line whatever the acidity or basicity of the solution : the cooperativity between the two functional groups -OH an greater than or equal to N in the excited state leads to apparently enhanced photoacidity and photobasicity of the molecule because of the coupling between proton and electron transfers. The high rate of the latter process in the excited state and in the transfer back to the ground state drives the proton transfer which is only limited by the ability of the water molecules to behave as proton accepters (in concentrated acidic media) or proton donors (in concentrated basic media). No excited state equilibrium is ever established. This kinetic scheme relying on coupled proton and electron transfers is also shown to be valid for 5-, 7-, and 8-hydroxyquinolines. Some differences, e.g., in quantum yield of 7-hydroxyquinoline, can be explained in terms of different relative proportions of the zwitterionic and quinonoid forms of the tautomer, depending on the position of the OH group. Moreover, this scheme may be transposable to other classes of bifunctional molecules undergoing phototautomerization.