With the recent interest in photochemically mediated atom transfer radical polymerization (ATRP), an interesting question arises: how significant are the photochemical processes in ATRP reactions that are supposed to be chemically controlled, such as initiators for continuous activator regeneration (ICAR) ATRP? A comparison of the rates of polymerization under ICAR ATRP conditions under ambient lighting and in the dark indicates negligible difference in the polymerization rate, under the conditions [MA]:[EBiB]:[TPMA*2]:[CuBr2]:[AIBN] = 300:1:0.12:0.03:0.2 in anisole 50% (v/v) at 60 degrees C, where TPMA*2 is 1-(4-methoxy-3,5-dimethylpyridin-2-yl)-N-((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)-N-(pyridin-2-ylmethyl)methanamine. This indicates that under typical ICAR conditions activator regeneration is almost exclusively due to the chemical decomposition of AIBN, not ambient lighting. To further investigate the effect of light on the activator regeneration, experiments were performed combining ICAR and photochemical processes in a 392 nm photoreactor of intensity 0.9 mW/cm(2). In this process, termed PhICAR (photochemical plus ICAR) ATRP, the overall rate of activator regeneration is the sum of the rates of activator regeneration by chemical (ICAR) decomposition of AIBN and the photochemical activator regeneration. At low AIBN concentrations (0.035 equiv with respect to ATRP initiator), the contribution of the photochemical processes in the 392 nm photoreactor is approximately 50%. At higher AIBN concentrations (0.2 equiv with respect to ATRP initiator), the contribution of photochemical processes to the overall polymerization drops to 15% due to the higher rate of chemically controlled processes.