The stability constants of the mixed ligand complexes formed by Cu 2+ , 2,2′-bipyridyl or 1,10-phenanthroline (= Arm), and the dianion of phosphonylmethoxyethane (PME 2- ), ethyl phosphonate (EtP 2- ), methyl phosphonate (MeP 2- ), or D-ribose 5′-monophosphate (RibMP 2 -) (= R–PO 3 2- ) were determined by potentiometric pH titrations in water containing 30 or 50% (v/v) 1,4-dioxane (I = 0.1 M, NaNO 3 ; 25°C). The corresponding results regarding water as solvent were taken from our earlier work. Previous measurements with simple phosphate monoesters, together with the present results for RibMP 2 -, were used to establish log versus straight line plots. With the aid of the equilibrium constants determined for the MeP 2- and EtP 2- systems it is shown that simple phosphonates, i.e., those without an additional binding site, fit also on the same straight lines. Therefore, it could be demonstrated with these reference lines that the Cu(Arm)(PME) complexes in all solvents have a higher stability than expected for a sole phosphonate Cu 2+ coordination. This increased stability is attributed to the formation of 5-membered chelates involving the ether oxygen present in the – CH 2 – O – CH 2 –PO 3 2- residue of PME 2- . The formation degree of the 5-membered chelates in the Cu(Arm)(PME) systems varies only between about 65 and 85% in the three mentioned solvents, despite the fact that the stabilities of the Cu(Arm)(PME) complexes increase by more than 1.8 log units by going from water to 50% dioxane-water. It is concluded that (i) such 5-membered chelates will also be formed in mixed ligand complexes of other metal ions in solvents with a reduced polarity, and (ii), more importantly, that the same interactions will also occur with the parent compound of PME 2 -, i.e. the dianion of 9-(2-phosphonylmethoxyethyl)adenine (PMEA 2 -), a compound which shows antiviral properties and for which the ether oxygen is important.