Despite the number and variety of their biological applications, the mechanisms of action of the photoactive naphthalenic imides have not yet been fully elucidated. In order to provide mechanistic insight, the photochemistry of several N-substituted 1,8-naphthalimides (NI) and 1,4,5,8-naphthaldiimides (NDI) has been studied using absorption and fluorescence spectroscopy and by laser flash photolysis (lambda(exc) = 355 nm). The lowest singlet state (S-1) is mainly pi pi* in nature for NI whereas pi pi* character predominates for the NDI. This difference exerts a profound effect on subsequent reaction mechanisms: upon irradiation, only the NDI molecules can undergo intramolecular gamma hydrogen abstraction. In the case of NP-III, a bishydroperoxy NDI derivative, this photoprocess (Phi = 0.03) leads to concomitant formation of an oxygen-centered radical (epsilon = 21 600 M-1 cm(-1) at 465 nm in acetonitrile) and release of the hydroxyl radical((OH)-O-.). All the compounds studied produce the triplet state (in acetonitrile, epsilon(T) approximate to 10 500-11 500 M-1 cm(-1) at 470 nm for NI and 485 nm for NDI). The quantum yield of intersystem crossing was determined to be close to unity except where intramolecular gamma hydrogen abstraction was possible (Phi(isc) ( 0.5). The triplet states were found to efficiently sensitize the formation of singlet oxygen (with S-Delta > 0.8 for NI and > 0.5 for NDI). In the absence of quenchers, the triplet states react with the ground-state of starting material via electron-transfer with a high rate constant [k = (4-6) x 10(9) and 5 x 10(8) M-1 s(-1) for NDI and NT, respectively] to give the radical anion and radical cation of the corresponding naphthalenic derivative. The high reactivity of the triplet states toward electron donors such as DABCO and their low ability for hydrogen abstraction are typical of a pi pi* configuration. These mechanistic photochemistry results are discussed with regard to the photobiological effects observed for these compounds and show that the actual reaction leading to biological damage will depend on the microenvironment of the naphthalenic molecule.