The hydroboration of crotonaldehyde with excess of diborane (in ether, 0°)afforded n-butanol as the principal product. A reaction mechanism involving a diborate type β-elimination was proposed and verified. The hydroboration of olefins with electro-negative substituents is also discussed. The elimination during hydroboration was greatly suppressed at lower temperatures, and the normal hydroboration product (butandiol) was obtained predominantly.