The naturally occurring polycyclic quinone hypericin possesses light-induced antiviral activity against the human immunodeficiency virus (HIV) and other closely related enveloped lentiviruses such as equine infectious anemia virus (EIAV). We have previously argued that hypericin undergoes a fast proton transfer reaction in its singlet state (J. Phys. Chem. 1994, 98, 5784). We have also presented evidence that the light-induced antiviral activity of hypericin does not depend upon the formation of singlet oxygen (Bioorg. Med. Chem. Lett. 1994, 4, 1339). It is demonstrated here that steady-state illumination of a solution containing hypericin effects a pH drop. When hypericin and an indicator dye, 3-hexadecanoyl-7-hydroxycoumarin, are both imbedded in vesicles, hypericin transfers a proton to the indicator within a time commensurate to its triplet lifetime. Proton transfer to the indicator is not observed when the indicator is protonated or when the system is oxygenated. Since hypericin is known to form triplets and to generate singlet oxygen with high efficiency, this latter result is taken to confirm triplet hypericin as a source, but not necessarily the only source, of protons.