Decaborane way shown to be a new hydroboration agent. Thus by reacting with deoaborane in tetrahydrofuran followed by alkaline hydrogen peroxide oxidation, camphene was converted into endo-camphenilanol and exo-oamphenaldehyde; Δ²-cholo-stene was converted into a mixture of 3α-, 2α- and 3,β-oholestauol and Δ⁵-cholestene into a mixture of 6α-oholestanol and cholestan-6-one. Cholestan-3-one was reduced by deoaborane to give a mixture of 3,β-and 3a-oholestanol. These results indicated that hydroboration with deoaborane proceeded by cis-addition with the attacking reagent entering the less hindered side of the double bond, was analogous to the ordinary hydro-boration with diborane. In the hydroboration of Δ⁵-cholestene, the reactivity of decaborane was remarkably lower than that of diborane. It is possible that the difference was due to the greater sterio requirement for the approaching of decaborane molecule toward the reaction site.