The preparation of the hydrophosphorylation catalysts succeeds via the metalation of dimesitylphosphane oxide and diphenylphosphane sulfide with potassium hydride in ethereal solvents such as tetrahydropyran (THP) and tetrahydrofuran (THF) yielding the tetramers [(thp)K-(OPMes(2))](4) (1a) and [(thf)(3){K(OPMes(2))}(4)] (1b) as well as [(thp)KSPPh2](infinity) (2) with a strand-like structure in the crystalline state. In ethereal solution these complexes very slowly degrade into KPAr2 and KE2PAr2 (E = O, S). The catalytic conversion of iPr-N=C=E' (E' = O, S) and of R-N=C=N-R (R = iPr, cHex) to the addition products Ar2P(E)-C(=E')-NHR (Ar = Ph, Mes; E = O, S; E' = O, S, NR) was studied in the presence of catalytic amounts of Ar2PEK (Ar = Ph, Mes; E = O, S). Steric hindrance prevents the addition of dimesitylphosphane oxide to N,N'-diisopropylcarbodiimide, whereas diphenylphosphane oxide and sulfide smoothly add to iPr-N=C=N-iPr yielding Ph2P(E)-C(=-N-iPr)-NHiPr (E = O, S).