Abstract The compounds (CH 3) 2 BC 2 H 5 and CH 3 B (C 2 H 5) 2 were prepared by reaction of (CH 3) 4 B 2 H 2 and C 2 H 4. The compounds were readily isolable in the pure state. A thorough characterization of these compounds is presented. Conclusions relevant to bonding in the analogous unsaturated methylvinylboranes are discussed in the light of the results of this investigation.