Zinc-iron oxide nanoparticles (ZnxFe3−xO4 and δ-ZnxFe1−xOOH) were successfully synthesized by room temperature chemical reaction of a solution containing ZnCl2 and FeCl2 in the presence of gelatin. The composition of products could be controlled by variation of the Zn/Fe mixture ratio of the starting material. ZnxFe3−xO4 nanoparticles were obtained from a solution with a high Zn/Fe ratio, whereas Zn-doped feroxyhyte (δ-ZnxFe1−xOOH) nanoparticles were obtained from a solution with a low Zn/Fe ratio. The ZnxFe3−xO4 nanoparticles were spherical with diameters of approximately 10 nm, and the δ-ZnxFe1−xOOH particles were needle-like with lengths of approximately 100 nm. Mossbauer spectra measured at room temperature indicated superparamagnetic behavior of the nanoparticles, whereas the magnetic components were observed at low temperature. The Zn content of the intermediate species ( $(\text {Zn}^{\text {II}}_{\mathrm {x}}\text {Fe}^{\text {II}}_{\mathrm {1-x}}\text {Fe}^{\text {III}}_{\mathrm {2}}\mathrm {O}_{4})$ ) plays an important role in the oxidation process. When the Zn concentration was high, the content of Fe2+ in the intermediate species was small, and Zn2+ prevented further oxidation of the nanoparticles. When the starting material had low Zn concentration, the amount of Fe2+ in the intermediate species became large and was rapidly oxidized into δ-ZnxFe1−xOOH while rinsing under the ambient atmosphere.