The Rp diastereomer of (-)-menthylmesitylphosphine, (Rp)-1, has been isolated with high configurational purity at phosphorus by fractional crystallization of an (Rp)-1/(Sp)-1 = 43/57 mixture from acetonitrile containing a trace of sodium acetylacetonate as a proton scavenger or by deboranation of the corresponding borane complex (Sp)-2 with diethylamine, thereby effecting the first resolution of a secondary phosphine chiral at phosphorus. The crystal and molecular structure of (Sp)-2 has been determined. The ready isolation of (Sp)-2 of 97% diastereomeric purity in 66% yield from an equilibrium (Rp)-2/(Sp)-2 = 28/72 mixture in n-hexane by second-order asymmetric transformation and its quantitative and stereospecific conversion under mild conditions into (Rp)-1 of similar purity augurs well for the future of the resolved secondary phosphines in stereoselective syntheses.