Iron-nickel alloys have been synthesized through thermal decomposition of nitrates aqueous solutions followed by H2 reduction. The effect of temperature [773 K to 973 K (500 °C to 700 °C)] over the oxide reduction kinetics has been studied. The results indicate that at 973 K (700 °C) and for a reaction time of 90 minutes, it was possible to obtain Fe-Ni alloys with distinct compositions. The X-ray diffraction analysis (XRD) of the initial oxide mixture has shown distinct peaks of iron and nickel oxides (Fe2O3, NiO, and Fe2NiO4). The same analysis for the reduced samples suggests that alloying has taken place. Indeed, the Rietveld analysis of the XRD patterns indicates the presence of two solid solutions—alpha (BCC) and FeNi3 (FCC), with varying mass percent depending on the initial Fe to Ni ratio, in accordance with the information contained in the Fe-Ni phase diagram. The calculated alloy average crystallite size has shown to be equal to 26 nm. Total magnetization measurements suggest a superparamagnetic behavior, which is a typical result for magnetic materials of considerable nanostructured content, in accordance with the expectation based on the quantitative evaluation of the XRD data.