The study presented in this paper is focused on the stabilization of metal oxide nanoparticles in aqueous suspensions in the form of soft magnetic materials, i.e. water based ferrofluids. Magnetic nanoparticles of Fe3O4, CoFe2O3, ZnFe2O4 and Mn0.5Zn0.5Fe2O4 were prepared by chemical coprecipitation in alkali medium. Fine granularity and spinel crystallinity properties were evidenced in all samples by Atomic Force Microscopy and X-ray diffractometry while superparamagnetic properties were investigated by means of vibrating sample magnetometry; significant coercitivity field was found only for cobalt ferrite nanoparticles. The stabilization of various types of magnetic grains in water was possible due to the sodium oleate coating shell – an organic molecule with biocompatibility features – enabling the producer to ensure the hydrophobic repulsion balance with magnetic attraction among nanoparticles. Various levels of saturation magnetization for the nanoparticles prepared by the same basic protocol sustain the possibility of producing diverse ferrofluids for various biomedical applications: magnetically targeted drug delivery, contrast agent in magnetic resonance imagistic, cancer experimental thermotherapy.