In this article, we extend our mechanistic study of the three-component radical photoinitiator system, consisting of methylene blue (MB), N-methyldiethanolamine, and diphenyliodonium chloride, by investigating the influence of oxygen on the rate of the consumption of MB dye. The mechanism involves electron transfer/proton transfer from the amine to the dye as the primary photochemical reaction. Oxygen quenches the triplet state of the dye, leading to retardation of the reaction. We used time-resolved steady-state fluorescence monitoring to observe the MB concentration in situ in both a constant oxygen environment and a sealed reactor as the dye is consumed via photoreaction. In the sealed reactor, we observed a retardation period (attributed to the presence of oxygen) followed by rapid exponential decay of the MB fluorescence after the oxygen was depleted. On the basis of the impact of the amine and iodonium concentrations on the fluorescence intensity and the duration of the retardation period, our proposed mechanism includes an oxygen-scavenging path-way, in which the tertiary amine radicals formed in the primary photochemical process consume the oxygen via a cyclic reaction mechanism. The iodonium salt is an electron acceptor, acting to reoxidize the neutral dye radical back to its original state and allowing it to reenter the primary photochemical process.