The addition reactions of dimethyltin dihydride with ethylene, butadiene, and cis- and trans-butene are described. With ethylene and butadiene, the products (CH3)2SnH(R) and (CH3)2SnR2 are fully characterized, where R = C2H5, or —CH2CH=CHCH3, the latter being the first butenyltin compounds to be reported. The butenes were virtually unreactive towards the dihydride. From a consideration of these and previous results, the possible reaction mechanism is discussed, and the tentative conclusion is reached that the rate-determining factor involves the formation of radicals from the tin dihydride.