UV irradiation of aqueous 4-hydroxyquinoline (4HQN) solutions results in the formation of the triplet states with the quantum yields 30%, 35%, and 7.5% in acidic, neutral, and basic solutions, respectively. In neutral solutions, the keto form is the major tautomeric structure for ground, excited singlet and triplet states of 4HQN. Triplet 4HQN reacts with amino acids tryptophan and tyrosine and with antioxidant ascorbate via the mechanism of electron transfer. The individual experimental and calculated UV-Vis spectra of different acid-base forms of 4HQN in the ground and triplet excited states, as well as reduced and oxidized forms of 4HQN, are reported. Under intense laser irradiation 4HQN undergoes biphotonic ionization, the excited singlet state being the precursor for photoionization.