The monohydroborations of dodeca-5,7-diyne, 2,7-dimethylocta-3,5-diyne, and 2,2,7,7-tetramethylocta-3,5-diyne with disiamylborane [bis(3-methyl-2-butyl)borane] proceed to place the boron preferentially at the internal positions of the diyne system. The organoboron intermediates upon protonolysis with acetic acid give the corresponding cw-enynes in high yields. Oxidation of the monohydroboration products with alkaline hydrogen peroxide affords the corresponding ,ß-acetylenic ketones in better than 70% yields. Dihydroboration of dodeca-5,7-diyne, 2,7-dimethylocta-3,5-diyne, and 2-methyldodeca-3,5-diyne with dicyclohexylborane followed by protonolysis produces the corresponding dr,di-dienes. For hindered diynes such as 2,2,7,7-tetramethylocta-3,5diyne, however, reduction to the diene has to be carried out stepwise. Thus, the diyne is first converted to the enyne via the hydroboration-protonolysis reaction. Subsequent hydroboration of the di-enyne followed by protonolysis with acetic acid affords dr,dj-2,2,7,7-tetramethylocta-3,5-diene. The hydroboration of conjugated dienes has been the subject of numerous investigations and has resulted in new routes to a large number of acyclic and cyclic organoboranes which undergo versatile chemical transformations.2 The hydroboration of conjugated diynes and the chemistry of the resultant organoboranes, on the other hand, has remained essentially unexplored.