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Nanoparticles of cobalt–zinc ferrite Co 1− x Zn x Fe 2 O 4 with x varying from 0 to 1.0 were prepared by a new method, the thermal decomposition of carboxylates of Fe(III), Co(II) and Zn(II). The obtained carboxylate precursor was characterized by thermal analysis and FT-IR spectroscopy. The precursor was annealed at 350, 600 and 1000 °C. It was found that the spinel cobalt–zinc ferrite was formed starting at 350 °C, but in mixture with simple oxides γ-Fe 2 O 3 , Co 3 O 4 and ZnO. At 1000 °C Co 1− x Zn x Fe 2 O 4 was formed quantitatively as a single, well-crystallized phase. The saturation magnetization of the samples annealed at 1000 °C decreased significantly with increasing Zn 2+ content from 83.93 emu/g ( x =0) to 4.92 emu/g ( x =1.0). At 350 and 600 °C the saturation magnetization had the same trend, even if there were contributions of other magnetic phases. Obtaining of spinel ferrite was evidenced by X-ray diffractometry and FT-IR spectrometry. Powder morphology was determined by scanning electron microscopy. Magnetic properties of the synthesized ferrites were investigated employing a conventional induction method.