The hydrogen bond strengths in the title isomorphous compounds as deduced from the infrared wavenumbers of the respective uncoupled OD stretches of matrix-isolated HDO molecules are discussed in terms of O w Á Á ÁO hydrogen bond distances, metal-water interactions (synergetic effect) and unit-cell volumes (repulsion potential of the lattice). The m OD stretching modes caused by guest ions (Cu 2+ and M 2+ guest ions matrix-isolated in M(HCOO) 2 AE2H 2 O and Cu(HCOO) 2 AE2H 2 O, respectively) are shifted at higher frequencies as compared to those in the host and the parent compounds, respectively. The values of these shifts (Dm) for the MAOH 2 Á Á ÁOCHOACu hydrogen bond systems formed when Cu 2+ guest ions are included in the structures of M(HCOO) 2 AE2H 2 O are considerably larger (i.e., weaker hydrogen bonds) as compared to those of the same type formed when M 2+ guest ions are included in the structure of Cu(HCOO) 2 AE2H 2 O. A variant of double matrix infrared spectroscopy (M(HCOO) 2 AE2H 2 O, M = Mg, Co, and Ni) with incorporated Cu 2+ ions up to 50 mol% additionally to matrix-isolated HDO molecules is proposed in order to deduce the cation distribution in mixed formates. The analysis of the infrared spectra reveals that the included Cu 2+ ions are localized at the two available lattice positions in the M(HCOO) 2 AE2H 2 O (M = Co and Ni) host lattices and preferentially at the M(1) sites (sites with a coordination through six formate oxygen atom) in the Mg(HCOO) 2 AE2H 2 O host lattice.