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The ultrafast excited-state dynamics of the fluorescent pigment yellow 101 (P.Y.101) and the closely related 1,1‘-naphthalazine, a nonfluorescent derivative that lacks the OH groups at the naphthyl rings, are studied combining femtosecond spectroscopy and high-level quantum chemical calculations. The observed ultrafast dynamics and the spectral signature of photoexcited 1,1‘-naphthalzine can be consistently explained with a previously proposed mechanism, suggesting fluorescence quenching via an optically forbidden n state. In contrast, for a description of the excited-state dynamics of P.Y.101, the expected simple absorption/fluorescence model is not adequate. Instead, besides fluorescence as the main decay channel of the excited-state population, ultrafast excited-state intramolecular proton transfer (ESIPT) and isomerization processes have to be considered for a complete understanding of the complex subnanosecond dynamics. Combining experiment and theory, the following kinetic model is derived:  upon photoexcitation a major part of the excited-state population decays via fluorescence from an enol-type isomer of P.Y.101, while a small part of the population undergoes ESIPT and fluoresces from a keto-type form. Furthermore, arguments are given that, to a minor extent, also trans−cis isomerization of the keto form takes place on the S1surface leading probably to a long-lived cis-keto form in the ground state. The remarkable photostability of this organic pigment is thus achieved by the interplay of different ultrafast nondestructive decay channels. [ABSTRACT FROM AUTHOR]