AbstractTris‐chloromethyl‐phosphine oxide, (ClCH2)3PO(I), is obtained by chlorination of (HOCH2)3PO with PCl5or (C6H5)3PCl2, and also by oxidation of (CICH2)3PO and (ClCh2)2(CH3)PO. High yields of tris‐(dialkyloxyphosphonly‐methyl)‐phosphine oxides, [RO2(O)PCH2]2PO (II) (RCH3, C2H5,iso‐C3H7,n‐C4H9, 2‐ ethyl‐hexyl), tris (alkyloxyphosphinyl‐methyl)‐phosphine oxides, [R2(O)PCH2]3PO(R = C6H5, CH3) are obtained by heating tris‐chloromethyl‐phosphine oxides, [(RO) (R′) (O)PCH2]3PO (R = C4H9, R′ C6H5) and tris‐(oxophosphoranyl‐phosphine oxides with phosphites, phosphonites and phosphinites, respectively, at 170–180°C for several hours. Compounds II possess an extraordinarily high absorption capacity. Thus a warm. 2% solution of II (R = C2H5) in benzene solidifies completely on cooling so that no benzene can be poured off.Tris‐dihydroxyphosphonyl‐methyl)‐phosphine oxide, [(HO)2(O)PCH2]3PO, obtained by hydrolysis of II (R  C2H5) with refluxing conc. HCl or by thermal decomposition of II (R iso‐C3H7) at 190°, titrates in aqueous solution as a hexabasic acid with breaks at pH = 4,4 (three equivalents) and pH = 10,7 (three equivalents). It forms crystalline salts with amines, alkali and alkaline earth metals, and is an excellent chelating agent.Th