The absorption spectra and fluorescence spectra of a coumarin derivative (1), designed to be a water-soluble photo-labile protecting group for caged compounds, were successfully observed in aqueous buffer solutions at pH 2.0-12.5 for the first time. Coumarin 1 is highly soluble (> 5 mM) in aqueous buffer at pH 7.2 and is stable in the dark at room temperature for up to a week. The pK(a) value for the protonation of anilino nitrogen in coumarin 1 was determined to be 4.5. Coumarin I has a fluorescence lifetime of 2.0 ns and undergoes intersystem crossing to the triplet state. Therefore, compound 1 can be used for the light absorbing part of the new water-soluble caged compounds, such as caged amino acids or caged nucteotides, which can release free amino acids or nucleotides, respectively, by excitation in the visible region.