A comprehensive electron magnetic resonance investigation was undertaken on a series of alkylmethylviologens which were solubilized into a D20 suspension of dipalmitoylphosphatidylcholine vesicles with and without added cholesterol. These samples were photoirradiated at 77 K with ultraviolet light, and the radical yield was determined through electron spin resonance spectroscopy. The location of the photoreduced species was determined by means of both deuterium electron spin-echo modulation (ESEM) spectroscopy at 4 K and proton electron nuclear double resonance (ENDOR) spectroscopy at 140 K. The effect of added cholesterol on the photoyield was correlated with the location of the photoreduced radical within the lipid bilayer. Addition of cholesterol disrupts the interfacial region of the vesicles, allowing increased water penetration which serves to enhance the radical yield. As the pendent alkyl chain length is increased, the photoreduction yield increases and deuterium ESEM and proton ENDOR show that the viologen moiety is "pulled" deeper into the bilayer. This is consistent with the interpretation that deeper solubilization decreases the rate of electron back-transfer which effects an increased photoreduction yield.