Abstract—A single transient species is observed in absorption following the flash irradiation of dilute deoxygenated aqueous alcoholic solutions of rhodamine B in hte pH range 5.5–7.0 The second‐order decay constant for this semiquinone radical is an approximate linear function of the prevailing hydrogen ion concentration, the transient persisting for a period of from several seconds in neutral solution to several hours at pH = 12.The slow rate of decay and its pH‐dependence are explained in terms of an electron or H‐atom transfer between the (unobserved) protonated and (observed) unprotonated semiquinone radical with simultaneous regeneration of hte D+>and production of the leuco‐dye DH:—D·+ DH·+→ D++ DHInsofar as the reactant concentrations are controlled by the acid dissociation constant Kaof DH.+the data are consistent with a value of 5.5±1.0 for pKaand a pH‐independent rate constant Koof 1.3±0.5×106