Acid-base equilibrium and photoinduced electron-ejection processes of N,N-dimethyl-1-aminonaphthalene (DMAN) in aqueous solution have been investigated by nanosecond laser photolysis and fluorescence measurements. Depending on the different pK(a) values for the ground (pK(a)(S-0) = 4.6), lowest excited singlet (pK(a)(S-1) = 2.5), and triplet (pK(a)(T-1) = 2.7) states, photoionization pathways varied with acidity of solution. Upon 266 nm laser excitation, the direct electron-ejection took place from the S-1 state of DMAN in aqueous solution at pH > similar to6.0, which was revealed to occur from the nonrelaxed S-1 state. At pH 2.0-4.0, proton dissociation took place in the S-1 state of DMANH(+) (protonated DMAN), which was followed by electron-ejection to produce DMAN(+.), In addition to the above ionization processes, a slow electron-ejection process was found to occur from the T-1 state of DMAN. The possible photoionization mechanisms of aqueous DMAN are discussed on the basis of kinetic analyses, thermochemical considerations, and electronic structures.