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Abstrart—The photochemistry of chlorpromazine (CPZ) with guanosine‐5′‐monophosphate (GMP) was studied as a model for the photoaddition of CPZ to DNA. Irradiation of CPZ with calf thymus DNA produced a product emitting at 520 nm, whereas with GMP emission was at 495 nm. HPLC separation of photolysis mixtures of [3H]CPZ with GMP and [14C]GMP with CPZ indicated that three photoadducts were formed. One of the adducts fluoresced at 500 nm and appeared to be the product detected but not separated by Fujita et al. (Photochem. Photobiol. 1981, 34, 101–105). A second adduct emitted at 460 nm, and the third was nonfluorescent. The photoadduct emitting at 500 nm was characterized by UV, fluorescence, and NMR to be an adduct from coupling of the C‐8 position of guanine to the C‐2 position of the phenothiazine ring of CPZ. The cation radical of CPZ (CPZ +) does not appear to be an intermediate since enzymatically generated CPZ + formed a product that eluted with a retention time close to that of the photoadducts, but did not emit at 520 nm.