Most visible-light photoinitiators are based on electron transfer processes and are comprised of two or more components. These initiators can lose effectiveness in viscous systems because the underlying reactions are diffusion controlled. In this contribution, the visible-light photoinitiator bis(cyclopentadienyl) bis[2,6-difluoro-3-(1-pyrryl)phenyl]titanium is characterized for polymerization of viscous systems and low light intensities. This compound absorbs visible light at wavelengths up to 550 nm, and does not rely on diffusion-controlled electron transfer reactions because it undergoes unimolecular decomposition. In contrast to trends observed for other photoinitiators, the effectiveness of the compound is found to increase markedly with the addition of protonic acids and with increasing system viscosity. For a given concentration of initiator and acid, a remarkably low optimal light intensity for effective polymerization is observed. The origins of these surprising results are discussed in terms of the mechanism of decomposition of the photoinitiator. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 611-618, 2013