A novel near-UV photoinitiator, diphenyliodonium 9-acridinecarboxylate (DPIOAC), was synthesized by metathesis, characterized, and studied. In media of different polarity, this compound showed different absorption and fluorescence profiles and photochemical properties. When irradiated with near-UV light, the anion absorbed the radiation and sensitized the photolysis of the iodonium cation. The active free radicals thus formed initiated polymerization of vinyl monomers. The facts influencing the form of the existence of the ion pairs influenced the rate and efficiency of the intra-ion-pair electron transfer and, thus, of the polymerization. This new photoinitiator had a much greater photoinitiation ability for polymerization than the previously reported diphenyliodonium anthroate, which has a carbon atom at the nitrogen position of DPIOAC.