The effects of deviation from stoichiometric composition on the structural and magnetic properties of cobaltferrite, Co_xFe_3−xO₄ (x = 0.2 to 1.0), are presented. Both CoFe₂O₄ and Fe₃O₄have a spinel crystal structure, and it might be expected that intermediate compositions would have the same structure. However, results show that synthesis via the ceramic method leads to the development of a secondary α-Fe₂O₃ phase. Both structural and magnetic properties are altered depending on the concentration of the α-Fe₂O₃ phase. Saturation magnetization is at a maximum for the stoichiometric composition (i.e., x = 1.0) and thereafter decreases with x. In attempts to achieve the properties required for application, the ceramic method offers the ability to selectively adjust the concentrations of both CoFe₂O₄ and α-Fe₂O₃ phases.