MnZn ferrite fibers are promising materials for technological applications. The Mn0.4Zn0.6-xCoxFe2O4 (x=0.0-0.4) ferrite fibers have been successfully prepared via the organic gel-thermal decomposition process using metal salts and citric acid. The structure, thermal decomposition process and morphologies of the gel precursors and fibers derived from thermal decomposition of these precursors were characterized by Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric and differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The saturation magnetization and coercivity of the prepared fibers were measured using vibrating sample magnetometer (VSM). The gel formed at pH 5-6 exhibits a good spinnability. The prepared ferrite fibers having a hollow structure are featured with diameters of 0.5-5 mu m and aspect ratios up to 1 x 10(5). It is found that the Mn0.4Zn0.6Fe2O4 fibers obtained at low calcination temperature (400 degrees C) show superparamagnetism due to grain size below the threshold value. With increasing calcination temperature and prolonging holding time, both the saturation magnetization and coercivity of these fibers increase with an increase in crystalline grain sizes and improvement of the crystallization. The substitution of Co for Zn has a remarkable influence on the magnetic properties of the MnZn ferrite fibers. The saturation magnetization and coercivity of the Mn0.4Zn0.6-xCoxFe2O4 ferrite fibers increase from 9.57 A m(2) kg(-1) and 2.74 kA m(-1) for x=0.0. to 22.31 A m(2) kg(-1) and 7.17 kA m(-1) for x=0.4, respectively. (c) 2008 Elsevier B-V. All rights reserved.