The addition of R3Si*H to olefins (hydrosilation) catalyzed by transition metal centers proceeds with retention of configuration as do Si*H-Si*D isotopic-exchange reactions. The data are accommodated by the general mechanistic conclusion that the initial process in these reactions involves stereospecific retentive interaction of the Si*-H bond with the reactive transition metal center to form a metal-silicon bond which then reacts in a variety of ways with complete retention of configuration in hydrosilation and isotopic exchange reactions. © 1969, American Chemical Society. All rights reserved.