The mechanism of hydrosilylation using the highly active precatalyst Karstedt's precatalyst (Ptx(()M(vinyl)M(vinyl))(y), (MMvinyl)-M-vinyl = divinyltetramethyldisiloxane) was investigated using extended X-ray absorption fine structure (EXAFS), small-angle X-ray scattering (SAXS), and ultraviolet-visible (UV-vis) spectroscopy. Contrary to previous reports suggesting colloidal catalysts, we find that regardless of the stoichiometric ratio of hydrosilane to olefin, the catalyst is a monomeric platinum compound with silicon and carbon in the first coordination sphere. The platinum end product, however, is a function of the stoichiometry of the reactants. At excess olefin concentration, the platinum end product contains only platinum-carbon bonds, whereas at high hydrosilane concentration, the platinum end product is multinuclear and also contains platinum-silicon bonds. The two end products can interconvert by adding additional aliquots of the stoichiometrically deficient reagent. An explanation of the "oxygen" effect is also given. In the absence of oxygen, hydrosilylation of certain olefins does not occur. Oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. Finally, we discuss the olefin isomerization reaction that may accompany hydrosilylation of terminal olefins. When the hydrosilylation reaction proceeds slowly;due to poorly reactive olefins, the olefin isomerization products become significant. EXAFS analysis of solutions after olefin isomerization has occurred shows the presence of platinum-platinum bonded compounds.