(2R,5S)-2-(2-Methoxyphenyl)-3-oxa-1-aza-2-phos-phabicyclo[3.3.0]octane(P-B)borane, C12H19BNO2P, is one of a series of new chiral oxazaphospholidine-borane complexes used as catalysts in the enantioselective borane reduction of ketones. The structure and the absolute configuration about the P atom were determined.