Based on new experimental data, a refined mechanistic picture of carbonyl hydrosilylation with hydrosilanes catalyzed by a cationic rhenium(V) oxo complex introduced by Abu-Omar is presented. A stereochemical analysis employing a silicon-stereogenic hydrosilane as a stereochemical probe revealed that both free silylcarboxonium ions and neutral rhenium(V) hydrides are key intermediates in the catalytic cycle. Moreover, nuclear magnetic resonance spectroscopic measurements and further control experiments shed light on the Si−H bond activation and the reactivity of the rhenium(V) hydride. These experiments also contributed to the understanding of the role of the acetonitrile ligand that is introduced with Abu-Omar's complex. Acetonitrile suppresses overreduction while without deoxygenation prevails.