The reaction of Me2AsNMe2 with Ph2AsH yields Ph2AsAsMe2, Me2AsAsMe2, Ph2AsAsPh2 and Me2NH. The course of the reaction was studied over a range of temperatures and concentrations in toluene-d8 solution 1H and 13C NMR spectroscopy. The NMR data indicate the initial formation of Me2NH and Ph2AsAsMe2. The diarsine subsequently undergoes symmetrization to give a mixture of the unsymmetric diarsine and the two symmetric diarsines. All attempts to isolate Ph2AsAsMe2 were unsuccessful. Exchange processes involving the .AsN< >NH and >AsAs< >AsAs< systems are very influential in determining the overall rate of reaction. Equilibrium constants were determined for the Me2AsAsMe2 Ph2AsAsPh2 and Et2AsAsEt2/Ph2AsAsPh2 systems. The results are compared with those of the analogous Me2AsH Me2AsAsMe2 system. © 1988.