Double labeling has been used under the endocyclic restriction test to show that transfers of phosphorus from oxygen to carbon in the conversions of lithio phosphinite 1 to alkoxy phosphine 8, of lithio phosphinite 2 to alkoxy phosphine 9, of lithio phosphinate 3 to alkoxy phosphine oxide 10, and of lithio phosphinite borane 4 to alkoxy phosphine borane 11 proceed in an intramolecular fashion. The transfers of stereogenic phosphorus in the conversions of lithio phosphinite (R)-5 to alkoxy phosphine (R)-12, of lithio phosphinate (S)-6 to alkoxy phosphine oxide (S)-13, and of lithio phosphinite borane (R)-7 to alkoxy phosphine borane (R)-14 proceed with retention of stereochemistry at phosphorus. These results rule out the classic in-line S(N)2 pathway and the geometrically equivalent in-line addition-elimination pathway for these endocyclic transfers of phosphorus. The most likely pathway for these nucleophilic substitutions at phosphorus is initial apical nucleophilic attack followed by pseudorotation and elimination of the apical alkoxy leaving group.