The absorption and the emission spectra of the four ionic species formed by 3-, 6-, and 7-hydroxyquinoline in aqueous solution have been measured, and the constants governing the prototropic equilibria between the ionic forms of each compound in the lowest singlet excited state have been estimated theoretically from the transition energies and experimentally from the change in fluorescence spectrum with hydrogen-ion concentration. It is found that the phenolic group of the hydroxyquinolines is more acidic, and the ring nitrogen atom more basic, in the excited than in the ground electronic state, as expected from the theoretical electronic charge redistribution on excitation derived from a corresponding-carbanion model. The overall velocity of the two-stage prototropic change in the excited state from the neutral molecule to the zwitterion of the hydroxyquinolines in ethanol or neutral aqueous solution is found to be slower than the rate of emission, but the corresponding one-stage processes in acid or alkaline solution are comparable in rate or faster.

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