Dirhodium(ll) tetraacetate in combination with catecholborane catalyzes the isomerization of alkenes and dienes. Effective isomerization occurs at 80-135°C with the use of only 0.1 mol % rhodium acetate. With 2-methyl-1,5-hexadiene the disubstituted double bond is preferentially isomerized. In addition, hydrogen transfer hydrogenation occurs with 1,4-cyclohexadienes. The mechanism of these reactions is proposed to involve organoborane addition across a Rh-0 bond which activates the catalyst for isomerization and hydrogenation.