AbstractSingle crystal x‐ray structures of the diastereomeric 2‐dimethylamino‐4‐phenyl‐2H‐1,3,2‐oxazaphosphorinane‐2‐oxides, 5c and 6c, have been determined. The pair has been prepared in a straightforward manner by reaction of 3‐amino‐3‐phenylpropanol with Mo2P(O)Cl2, and separated into pure forms by column chromatography. Crystal data of 5c: space group P21/n, a = 11.087(3), b = 6.087(6), c = 18.465(6)Å, β = 98.40(3)°, R = 0.035 for 1845 reflections. Crystal data of 6c: space group P21/n, a = 11.176(2), b = 6.893(2), c = 16.673(3)Å, β = 95.29(1)°, R = 0.037 for 1253 reflections. The fast‐migrating 5c is cis (Ph and P = O group cis) with the configuration of 2RS, 4SR. The slow migrating 6c is trans with the configuration of 2RS, 4RS. In both structures the 1,3,2‐oxazaphosphorinane rings are chair like with the P‐end essentially flattened, The skeleton made up with 4‐phenyl‐1,3,2‐oxazaphosphorinane is essentially the same. With a conformationally demanding 4‐phenyl substituent, 5c has an axial Mc2N whereas 6c has an equatorial Me2N. For 6c, the exocyclic P‐N bond has a partial double bond character the geometry about exocyclic N is planar. On the other hand, for 5c, the geometry about exocyclic N deviates considerably from planarity, the sum of the angles around N being 348.1°. In the crystalline state, the screw‐related 5c molecules are hydrogen bonded whereas in the crystalline state, the centro‐symmetrically related 6c molecules are paired‐up by h