Recently, the addition of negatively charged liposomes was shown to increase the quantum yield of the photocatalytic reduction of 5,5'-dithio(2-nitrobenzoic acid) (H2DTNB) to 2-nitro-5-thiobenzoic acid (H2NTB) by triethanolamine using meso-tetra(4(N-methylpyridinium)porphyrinato zinc chloride as photosensitizer. In this work, we investigate in detail the kinetics of this photocatalytic reaction both in homogeneous solution and at the surface of negatively charged liposomes, to unravel the effects of liposomes on the mechanism of the photoreaction. In homogeneous solution, the reaction is initiated by oxidative quenching. Both static (singlet) and dynamic (triplet) quenching of the photosensitizer contribute to the formation of the photoproduct. In these conditions, the reaction is limited by the low efficiency of reductive regeneration of the photosensitizer, compared to charge recombination. Upon adsorption of the positively charged photosensitizer to the negative surface of the liposomes, however, both static and dynamic oxidative quenching become ineffective due to electrostatic repulsion of the dianionic DTNB2- from the negatively charged membrane. In such conditions, photoreduction occurs via reductive quenching, showing that the addition of liposomes can truly modify the mechanism of photocatalyzed redox reactions.