Ferromagnetic nickel-copper alloys can be charged with cathodic hydrogen in high concentrations whereby they loose their ferromagnetic behavior. Measurements of magnetization of these alloys show a quicker absorption of hydrogen compared to nickel. The interstitial occlusion of hydrogen gives rise to a reversible lattice expansion of some percent (without change of structure). During the desorption there appear on the studied alloys (10 to 35%) simultaneously two different values of lattice parameters. This is due to the transition between two NiCu-H phases of different H-con-centrations (β- and α-phase). With increasing Cu-content a decreasing lattice expansion of the hydrogen-rich non-ferromagnetic phase is found. This expansion up to a Cu-content of 35% is proportional to the mean magnetic moment per atom of the corresponding hydrogen-free alloy. The comparison with the quantity of absorbed hydrogen suggests further - in analogy to Vegards law for mixed crystals - proportionality between lattice expansion of the β-phase (as function of the Cu-content) and the corresponding atomic ratio hydrogen/metal. The results can be understood as a decrease of the solubility of hydrogen caused by the increased filling of the d-holes by s-electrons of Cu. - Further X-ray studies showed an overcharging effect by an additive continuous increase of the lattice parameters of the β-phase.