Me2AsH reacts irreversibly with Me2AsNMe2 and Me2AsNMe2•BH3 to give good yields of Me2AsAsMe2. The nitrogen-containing product is Me2NH or Me2NH•BH3, respectively. These reactions have been followed by using multinuclear (1H, 11B, and 13C) NMR spectroscopy to elucidate the respective reaction mechanisms. Although the Me2AsH/Me2AsNMe2 reaction proceeds faster initially, the overall rate of reaction is slower than that for the Me2AsH/Me2AsNMe2•BH3 reaction. This is a consequence of the presence of inhibiting exchange reactions in the Me2AsH/Me2AsNMe2 system that are absent when Me2AsNMe2 and Me2NH are bound to BH3 in the Me2AsH/Me2AsNMe2•BH3 reaction. A detailed NMR study of the exchange reactions involving the >AsN>NH, >AsAs>AsH, and >AsN>AsAs< systems is also discussed with respect to the relative impact of those systems on the rate of formation of Me2AsAsMe2. © 1986, American Chemical Society. All rights reserved.