The dissymmetric chelating agent (RR,SS)-ortho-phenylenebis(methylphenylarsine) has been resolved into its enantiomers by fractionally crystallising internally diastereoisomeric palladium complexes containing the ditertiary arsine and optically active ortho-metallated dimethyl(α-methylbenzyl)amine. The full procedure is described and constitutes the first satisfactory route to optically active ditertiary arsines containing asymmetric arsenic atoms. The optically active ditertiary arsines, α(589 nm, CH₂Cl₂)± 95°, are air-stable crystalline solids, m.p. 75–76 °C, and the absolute configuration of the enantiomer with a positive rotation at the sodium D line (589 nm) has been established as RR by a single-crystal X-ray determination of the structure of the internally diastereoisomeric palladium complex containing (S)(–)-dimethyl(α-methylbenzyl)amine. The meso diastereoisomer undergoes a halide-ion assisted epimerisation in acidic methanol solution and may be converted, under suitable conditions, into the less soluble racemic diastereoisomer thereby effecting a second-order asymmetric transformation between the diastereoisomeric ditertiary arsines.