The detailed triplet state characteristics of 2-(4-acetylphenyl)isoindolin-1-one (kINP), a N-phenylphthalimidine (PPI) derivative, have been studied in fluid solution at room temperature. The attachment of an acetyl group to the N-phenyl moiety of PPI has permitted to enhance the intersystem crossing quantum yield, generally low for such compounds. Upon 308-nm laser flash photolysis of kINP in acetonitrile, a triplet-triplet transition has been evidenced (lambda(max) = 440 nm). Further characterization of this transient at 440 nm gave a lifetime tau = 11 mus, a molar absorption coefficient epsilon = 22 000 M-1 x cm(-1), and an intersystem crossing quantum yield of 0.89. Moreover, a pipi* nature has been found for this triplet state that lies at ca. 290 kJ x mol(-1) above the ground state. In addition to providing fundamental information on the triplet state properties of PPI derivatives, its importance during a photobiological process has been evidenced. kINP is the key compound involved in thymine dimers formation during the photosensitization of DNA by indoprofen, a nonsteroidal antiinflammatory drug.