Ferrites with the composition Mg1−xCOxFe2O4(0<x≤0·33) were synthesized by solid state reaction between MgO,α-Fe2O3 and CoFe2O4 at 1300°C. The compounds were analyzed from the results of x-ray diffraction, magnetic hysteresis and initial susceptibility (χ) studies. All the compounds of the series were found to be monophasic with no appreciable change in their lattice parameter. The saturation magnetization and coercive field (Hc) values increased with increasing cobalt ferrite content. Similarly, the shapes of theχ-T curves and temperature variations ofHc values indicated that the magnetic behaviour changes from multi-domain to single domain with increasing cobalt content. The results are similar to those observed for the (MgFe2O4)1−x (Co3O4) system.