Treatment of 1-alkynylphosphines with a rhodium catalyst in 1,4-dioxane/H2O at reflux provides (E)-1-alkenylphosphine oxides in good yields with perfect stereoselectivity. The reaction proceeds as follows. Oxidative addition of 1-alkynylphosphine to rhodium followed by hydrolysis yields the corresponding terminal alkyne and diphenylphosphine oxide. Rhodium-catalyzed hydrophosphinylation of the terminal alkyne then proceeds to afford (E)-1-alkenylphosphine oxide.