To assess whether kinetic analysis of ethylene hydrosilylation would shed light on the rate determining transition state or would instead be overshadowed by mass transport limitations, we determined the kinetic isotope effect. The formation of the σ-adduct, which is the only step preceding Si-H cleavage, is not associated with any notable weakening of the Si-H bond and therefore does not show a significant equilibrium isotope effect. Consequently, the experimental kinetic isotope effect (KIE) could be determined by direct competition of HSiEt3 and DSiEt3, which ensures higher accuracy for a heterogeneous (light-mediated) reaction than would be possible for the comparison of absolute rate constants. Determination of the H:D ratios for tetraethylsilane formed in the presence of both HSiEt3 and DSiEt3 furnished a KIE of 1.85 ± 0.24 for light-mediated hydrosilylation and 1.90 ± 0.25 for thermal hydrosilylation with Rh(II)-3 (Table -S25). The KIE was identical within experimental error for the light-mediated and the dark reaction. A KIE of 1.9 ± 0.25 substantiated that Si-H cleavage takes place in the turnover limiting step, which is the case for both reaction pathways we have outlined. Furthermore, the experimental kinetic isotope effect is in excellent agreement with the theoretically predicted value of 1.9 for the dark reaction (Figure ). Notably, because the experimental intermolecular KIE is not substantially lowered compared with theoretical predictions, mass transfer of the silane through the MOF pore network is not limiting the reaction rate.