The sensitized polymerization of synthetic lipid bilayers via visible-light irradiation of membrane-bound cyanine dyes is reported. Visible-light-initiated polymerizations were successful with liposomes composed solely or partly of either 1-palmitoyl-2-[10-(2’,4’-hexadienoyloxy)decanoyl]-sn-glycero-3-phosphatidylcholine (mono-SorbPC), 1,2-bis[10-(2’,4’-hexadienoyloxy)decanoyl]-sn-glycero-3-phosphatidylcholine (bis-SorbPC), or 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphatidylcholine (DenPC). The ballasted sensitizing dyes reported here were conveniently incorporated into liposomes. The sensitizing dye N,N’-dioctadecyl-3,3,3’,3’-tetramethylindocarbocyanine perchlorate, DiIC(18)3, was used in many of these studies because its absorption properties were especially well matched to the spectral output of the light source. The lipid product from the sensitized photoconversion of mono-SorbPC was transesterified and analyzed by size exclusion chromatography, which revealed the presence of a polymer with a relative number-average degree of polymerization of 10(2). This value is similar to those reported for the AIBN-initiated polymerization of mono-SorbPC liposomes. The sensitized polymerization process is dependent on the presence of oxygen, and spin-trapping experiments demonstrated the formation of superoxide anion. The indivdual effects of pH, light intensity, and temperature suggest that the polymerization process is initiated by electron transfer from the dye excited state to oxygen, to yield superoxide anion, which in aqueous media combines to form hydrogen peroxide. In addition, it was shown that the monomeric lipids could be polymerized by addition of hydrogen peroxide at the experimental temperatures. These data suggest that the dye-sensitized polymerization is mediated by the formation of hydroxyl radicals in the immediate vicinity of the lipid bilayer. This polymerization chemistry appears likely to be compatible with oxygen-dependent systems such as living tissues.