Abstract—Sanguinarine, a commercial drug exhibiting antimicrobial and antitumor properties, was studied with respect to its basic photochemical characteristics and also with regard to its phototoxicity to mosquito larvae (Aedes atropalpus). Sanguinarine proved to be clearly phototoxic to larvae, with an LD50of 0.096 mg/mL with near UV exposure as compared with 23.3 mg/mL without. Flash photolysis experiments enabled the study of the triplet state of sanguinarine to be undertaken. Quenching by oxygen occurs with a rate constant of 6 × 109M‐1s‐1time‐resolved emission studies indicate that sanguinarine produces a significant amount of singlet oxygen (ΦΔ= 0.16) as does the isoquinoline alkaloid, berberine (ΦΔ= 0.25). These values represent the first direct quantitative measurements of photosensitization parameters of these compounds. Additionally, sanguinarine exhibits efficient electron donation properties, undergoing reaction with methyl viologen with a rate constant>1010M‐1s‐1but is a poor electron acceptor. Phototoxicity of sanguinarine can thus be explained in terms of its photosensitiz