Abstract Fluorescein radical anions were generated in situ by controlled potential electrolysis with a mercury pool electrode. Analysis of the time dependence of the polarographic currents of a partially electrolyzed solution revealed that two radical anions disproportionated into one original fluorescein anion and one electroinactive entity. The reaction rate was proportional to the square of the radical concentration. The second-order rate constant was a linear function of hydrogen ion concentration in the pH range of 11.1–14 (ionic strength=1 mol dm −3). The kinetic results were interpreted on the basis of a reaction scheme consisting of two parallel paths, direct disproportionation and hydrogen-ion catalyzed disproportionation. An activation energy of 63 .6 kJ mol −1 was obtained for the first path. Existence of other path(s), e.g., disproportionation between two protonated radical anions, at lower pH values was suggested.