The ferrofluids based on Zn_xFe_3-xO₄ (0.01 ≤ x ≤ 0.8) nanoparticles have shown stabilization near therapeutic temperature (~42 °C) in the presence of external AC magnetic fields similar to that of tetravalent ion substituted magnetites. These nanoparticles were prepared using microwave refluxing technique. Rietveld analysis of X-ray diffraction patterns of the as prepared samples confirmed their single phase mixed spinel type structure. It further suggests that all the Zn²⁺ ions occupy tetrahedral voids. The spherical shaped particles had sizes in the range of 3–10 nm, as observed under transmission electron microscopy. X-ray photoelectron spectra have confirmed the presence of Fe²⁺, Fe³⁺ and Zn²⁺ ions in the samples. The Zn_0.2Fe_2.8O₄ sample displayed the highest saturation magnetization value (~65 A m² kg^-1) amongst all the samples. As the concentration of Zn increased, beyond x > 0.3, the value of saturation magnetization continuously diminished due to the dominance of the nonmagnetic ion. Fe²⁺ ions occupying both the sites was confirmed from room temperature Mossbauer spectroscopy.