We chose the α-ethereal radical addition to 1 as a model reaction (Figure ). As previously discussed in Figure , the C-H bond activation of cyclic ether is the ratedetermining step of this reaction so the overall reaction rate The kobs values were measured under a modified reaction condition where cyclic ether is the limiting reagent, as shown in Figure . Figure shows the percentage change of [C-H] ([C-H]/[C-H]0) as a function of reaction time for methyl 1,3-dioxlane. [C-H] was monitored using 1 H-NMR (Figure ). The plot of ln([C-H]/[C-H]0) vs reaction time shows a linear relationship over 9 hours, indicating a pseudo-first-order reaction mechanism. kobs is measured to be 4.2 x 10 -5 s -1 from the slope of the linear fit. The change of [1] and [C-H]0 did not alter kobs, but reduced catalyst loading by 50% decreased kobs, by ~50%. In addition, the lowered light intensity also lowered kobs. All above observations are consistent with the reaction rate model in Figure , where kobs is proportional to [CdS*] but independent on [1] or [C-H]0.