1,2,3,4,5,6-Hexahydrophosphinine oxides with exocyclic P-function in position 3 can be obtained in a diastereoselective manner by the two-step conversion of 1,2-dihydrophosphinine oxides. Michael addition of the > P(O)H species on the α,β-double-bond of the starting P-cycle is followed by catalytic hydrogenation of the 1,2,3,6-tetrahydrophosphinine oxides so obtained. One of the bis(phosphine oxides) served, after deoxygenation, as a suitable bisphosphine ligand in complexation with PtCl2(PhCN)2. The different configuration of the ring P atom in the precursor and in the ligand suggested that the deoxygenation took place with inversion.