(i) prior literature reports state that trialkylsilanes are unsuitable substrates for hydrosilylation via a radical chain mechanism, (ii) no reaction was observed when we replaced 3 with AIBN (Figure ), and (iii) an on-off experiment showed substantial differences in the reaction rate in the presence and absence of light (Figure ). The active Rh(II) catalyst is generated by photolysis of a Rh(III)-Me pre-catalyst during which methyl radicals are generated. We could exclude that methyl radicals contribute noticeably to the observed reactivity since no decrease in reaction rates was observed when re-used catalyst (from which no methyl radicals can be released) was used to catalyze olefin hydrosilylation (Figure -S6). Based on detailed EPR analyses, labeling and trapping experiments, kinetic data, as well as solution and solid-state NMR experiments, which we will discuss below, we propose the catalytic cycles depicted in Figure , which are corroborated by quantum-chemical calculations. Our proposal includes two different reaction pathways for the rate-limiting transfer of silyl radical from the silane to the olefin, of which one leads to the generation of free silyl radicals, while the second pathway achieves a direct silyl radical transfer to the olefin.