Photophysical properties of fluorenone and 12 aminofluorenone derivatives were investigated systematically by fluorescence quantum yield and picosecond lifetime measurements in cyclohexane, benzene, acetonitrile, and ethanol. Aminofluorenones underwent efficient deactivation in ethanol, except fur the 1-aminofluorenone derivatives. An appreciable deuterium isotope effect on the radiationless deactivation (k(nr)) was observed in all aminofluorenones. The largest effect was observed for 4-aminofluorenone (k(nr)(EtOH)/k(nr)(EtOD) = 1.9), while the smallest was for 1-aminofluorenone (k(nr)(EtOH)/k(nr)(EtOD) = 1.2). The radiationless deactivation in ethanol was concluded to be induced through an intermolecular hydrogen bond between the hydroxyl hydrogen of ethanol and the carbonyl oxygen of aminofluorenones. The possibility of a twisted intramolecular charge-transfer state in the case of 1-(dimethylamino)fluorenone was explored by semiempirical molecular orbital calculation.