In order to compare the kinetic profile of light-mediated and thermal hydrosilylation with Rh(II)-3, we attempted numerous in-situ NMR experiments. Unfortunately, duplicate measurements frequently gave rise to notably different conversion rates (Figure ). During our search for the source of the rate variation, we employed increasingly rigorous means of excluding oxygen during the synthesis of active catalyst Rh(II)-3. When the reaction was set up in an argon-filled glove box with freeze-pump-thaw deoxygenated benzene we obtained Rh(II)-3 that gave rise to only 4% conversion in hydrosilylation. Metalloradical Rh(II)-3 is itself an effective oxygen trap, so we recycled the benzene for subsequent syntheses of the active catalyst. Rh(II)-3 from the first recycle achieved 2% conversion and Rh(II)-3 from the second recycle gave only 0.3% conversion, compared to ~20% conversion for Rh(II)-3 prepared with benzene that was only sparged with argon. To test whether the addition of small amounts of oxygen during the preparation of Rh(II)-3 increased the catalyst's hydrosilylation performance, we introduced controlled amounts of air (Figure ). Two plateaus were observed that corresponded to the addition of ~1 equivalent of O2 and more than 10 equivalents of O2 per Rh(II) center, respectively. While addition of 10 -33 equivalents of O2 furnished Rh(II)-3 that was able to achieve close to 50% conversion, Rh(II)-3 that was prepared in air was substantially less active (Figure ). We confirmed that oxygen rather than any other components of non-dried air was responsible for the rate enhancement by repeating key experiments with dried O2 gas (Figure ). In a series of control experiments (Figure -S15), we determined that i) while O2 must be present during active catalyst formation, the presence of O2 during the use of Rh(II)-3 in hydrosilylation is detrimental, and that ii) O2 and light must be present simultaneously. We knew from prior EPR experiments that the exposure of Rh(II) to air in the absence of light furnished Rh(III)-superoxide 9. We confirmed that superoxide formation is reversible in the presence of light, and Rh(II)-3 could be quantitatively recovered after 20 h of illumination (Figure ). Furthermore, Rh(III)-superoxide 9 was unable to promote olefin hydrosilylation. While Rh(III)-superoxide formation was not directly responsible for the beneficial effect of O2, we speculated that 9 may be converted into Rh(III)-hydroperoxide 10 in the presence of both light and O2. A light-promoted HAT from the benzene solvent to 9 would give rise to 10 and phenyl radicals during the preparation of the active catalyst. In support of such a pathway, mass spectrometric analysis of the reaction solvent led to the detection of biphenyl-h10 when Rh(II)-3 formation was carried out in benzene and biphenyl-d10 when benzene-d6 was used as a solvent (Figure ). Fur-thermore, FTIR analysis of Rh(II)-3 generated in the presence of 10 equivalents of O2 furnished a band at 829 cm -1 which is in good agreement with an O-O stretch of 823 cm - 1 for a reported rhodium-hydroperoxide (Figure ). The band in question was absent for Rh(II)-3 generated in airfree solvent and increased in intensity when 50 equivalents of O2 were present during Rh(II)-3 formation. We thus propose that diamagnetic Rh(III)-hydroperoxide 10 is formed alongside Rh(II)-3 when the active catalyst was prepared in the presence of a limited amount of O2. The beneficial effect of the converting a small number of Rh(II) centers to polar Rh(III)-O-OH groups raised the possibility that ethylene hydrosilylation may occur via a polar transition state, which is explored in detail in later sections. For consistency with our earlier work, kinetic experiments and experimental activation barriers presented in the later part of this work were determined for an oxygen content that corresponds to the first plateau in Figure . EPR spectroscopic characterization of reaction intermediates presented in the main text refer to rigorously deoxygenated samples in order to minimize the signal intensity of 9. However, all spectroscopic observations were also carried out for samples with varying oxygen content to verify that that all intermediates discussed below (but no additional intermediates) were likewise generated in the presence of oxygen (see SI for detailed discussion).