Laser Ñash photolysis and pulse radiolysis have been used to study the triplet and the free radical states of Rhodamine 123. Using acridine and xanthone as sensitisers, the molar absorption of the triplet state at 395 nm was found to be 1.2 ] 104 dm3 mol~1 cm~1 in both aqueous and ethanolic solutions. No triplet state of Rhodamine 123 was observed on direct excitation in aqueous solution, indicating a triplet intersystem crossing yield of less than 0.002. The formation and decay of the excited singlet state at 420 nm was observed during the laser pulse. One-electron oxidised and reduced forms of Rhodamine 123, attributed to the radical cation and radical anion with absorption maxima at 450 and 390 nm, respectively, were produced. The radical cation formed at pH 2.5 (designated was shown to deprotonate to yield Both the cation RhH~2`) R h ~(pK a \ 5.7). and anion radicals of the dyes may be formed either following photoionisation of Rhodamine 123 or, in photoredox reactions involving Rhodamine 123. Photoionisation was observed and appeared to be neither solely mono-or biphotonic. The maximum quantum yield for the monophotonic process was estimated to be O0.005. Direct excitation of Rhodamine 123 to produce signiÐcant yields of the dye triplet state and, subsequently, singlet oxygen is an unlikely mechanism for photosensitised cell killing. Accumulation of Rhodamine 123 in mitochondria may account for the apparent e †ectiveness of the dye as a photosensitiser, even in inefficient photoprocesses.