A novel one-pot reductive methodol. for the synthesis of optically pure tert-butylmethylphosphine-borane is reported. The prepn. uses as the starting material tert-butylmethylphosphinous acid-borane, which is available in both enantiomeric forms from cis-1,2-aminoindanol and tert-butyldichlorophosphine. The process is based on the redn. of a mixed anhydride, the configurational stability of which has been studied in several solvents and temps. Tetrabutylammonium borohydride was the best reducing agent allowing for the development of a practical process. To demonstrate the utility of the new methodol., the product obtained in this manner was used in the prepn. of Quinox-P*.