Abstract Fluorescence quantum yields Φ f and corrected fluorescence spectra were determined for fluorescein over a wide pH range (10 M H 2SO 4 to 0.01 M NaOH) in aqueous solution. The results were interpreted by comparison with those obtained for the very similar dye 6-hydroxy-9-phenyl-fluoron (HPF) which lacks the fluorescein carboxyl group. We find for the fluorescein cation Φ f = 0.9−1, for the neutral molecule Φ f = 0.20−0.25, and for the monoanion Φ f = 0.25−0.35. The neutral molecule and monoanion have identical fluorescence spectra: an emission with maximum at 515 nm, extending to about 700 nm. (The dianion also has a maximum at 515 nm but has a more narrow emission band.) The fluorescein cation dissociates in the excited state at an acidity corresponding to that of ≈3 M H 2SO 4. A detailed scheme of the protolytic reactions of fluorescein in the excited state is presented.