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This paper presents a novel measurement technique for the broadband electromagnetic characterization of diluted suspensions of magnetic nanoparticles in aqueous solvents, the so-called water ferrofluids, which are of particular relevance for biomedical applications. The technique relies on the possibility of turning off the magnetic response of the suspended nanoparticles, without affecting the electric response of the ferrofluid, by applying a polarizing magnetostatic field of proper strength. In such a way, two different datasets are collected: 1) the ON data (i.e., with the magnetostatic field applied), which only depend on the electric response of the investigated ferrofluid and 2) the OFF data (i.e., without the magnetostatic field applied), which depend on both the electric and magnetic responses of the ferrofluid. Therefore, the presented strategy enables the physical separation, at the measurement stage, of the electric response from the magnetic one, hence, a more reliable and accurate estimation of this latter, even in the case of strongly dielectric and weakly magnetic samples, as happens for diluted water ferrofluids. To implement the strategy a custom-made measurement cell has been designed and realized, by which an exhaustive measurement campaign, on a commercially available water ferrofluid, has been carried out. The obtained results show the effectiveness of the proposed strategy and its ability to estimate satisfactorily the magnetic response up to nanoparticle concentrations of a few milligrams per milliliter of solution, which are the concentrations of actual interest in biomedical applications.