Excited-state intramolecular proton-transfer (ESIPT) and singlet-oxygen (O-1(2)) quenching activities of intramolecularly hydrogen-bonded 1-(acylamino)anthraquinones have been studied by means of static and laser spectroscopies. The ESIPT shows a substituent effect, which can be explained in terms of the nodal-plane model. The ESIPT activity positively and linearly correlates with their O-1(2) quenching activity. The reason for this correlation can be understood by considering ESIPT-induced distortion of their ground-state potential surface and their encounter complex formation with O-1(2). Intramolecularly hydrogen-bonded hydroxyanthraquinones found in aloe also show a similar positive and linear correlation, which can be understood in the same way.