The photophysical and photochemical properties of the UV-A–absorbing phototoxic drug cyamemazine (CMZ) (2-cyano-10-(3-[dimethylamino]-2-methyl-propyl)-phenothiazine) have been investigated in neutral buffered aqueous solutions. The transient absorbances of the hydrated electrons, of the first excited triplet state (³CMZ*) with a characteristic absorption band peaking at 420 nm and of the radical cation (^·CMZ ) (maximum absorbance at 500 nm) have been observed by 355 nm laser flash spectroscopy of deaerated solutions. All these transient species are formed by monophotonic processes and react with oxygen. Bimolecular reaction rate constants of ^·CMZ and ³CMZ* with O₂ are 2 × 10⁷ M⁻¹ s⁻¹ and 4 × 10⁹ M⁻¹ s⁻¹, respectively. The ³CMZ* reacts only sluggishly (reaction rate constant, 9 × 10⁶ M⁻¹ s⁻¹) with tryptophan chosen as a Type-I photodynamic substrate. Steady-state irradiations with 365 nm light demonstrate that CMZ is rapidly photolyzed (quantum yield, 0.04) in O₂-saturated solutions leading to oxidation of the sulfur atom and of the side-chain nitrogen of CMZ. This photoproduct (2-cyano-10-(3-[dimethylamino, N-oxide]-2-methyl-propyl)-5-oxide–phenothiazine), is a good Type-I and Type-II photodynamic photosensitizer producing singlet oxygen in high yield (∼0.45) and could play a major role in the phototoxicity of CMZ.