Abstract <p>This paper presents the results of a study of a ferromagnetic liquid containing aggregates of dispersed-phase nanoparticles. The targeted synthesis of a ferrofluid based on Fe₃O₄ nanoparticles in a kerosene medium stabilized with oleic acid and containing magnetite particle aggregates approximately 5 μm in size is described. The various physical properties of the resulting colloidal system were investigated, including electrical, magnetic, rheological, optical, and thermal properties, as well as microwave properties within the 8.2–12 GHz frequency range. It was found that under the influence of an external magnetic field, there is a significant quantitative change in the studied parameters of the medium. It was also found that these parameters are influenced by the concentration of the aggregates contained in the samples.</p>