The one-electron oxidised (radical cation) and one-electron reduced (radical anion) forms of the photosensitiser rose bengal have absorption maxima at 470 nm (molar absorption coefficient 21 100 dm3 mol–1 cm–1) and 420 nm (molar absorption coefficient 37 600 dm3 mol–1 cm–1), respectively. The radical anion (RB˙–) undergoes electron-transfer reactions both with oxygen (k= 1.5 × 108 dm3 mol–1 s–1) and diethylenetriaminepentaacetic acid (DTPA)(k= 8.6 × 108 dm3 mol–1 s–1). Rose bengal reacts both with the solvated electron and the semireduced nicotinamide adenine dinucleotide radical (NAD˙) to give RB˙– with second-order rate constants of 3.1 × 1010 and 9.6 × 108 dm3 mol–1 s–1. The one electron-reduction potential E17 for rose bengal, measured via pulse radiolysis, was found to be –511 mV.