Pure and cobalt–ion-containing nickel ferrite samples Ni1–xCoxFe2O4 (x = 0, x = 0.01, x = 0.05 and x = 0.1) were prepared by using the sol–gel auto-combustion method. To form the desired structure after the auto-combustion process, the samples were annealed in air at 800 ◦C for 1 h, followed by slow cooling in air (2.5 ◦C/min) or rapid cooling by quenching in water. The influences of cobalt ion addition and the cooling rate on the electronic structure, morphology and electrical properties of Ni–Co ferrite were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The gas-sensing properties of pure and cobalt–ion-containing nickel ferrites were studied by using acetone as a test gas. The gas-response of nickel ferrite samples was strongly influenced by the cooling rate and cobalt addition. The effect was attributed to a change in the Ni3+ concentration and a restriction of the transduction of the chemical signal into an output electrical signal. It was demonstrated that the gas response of Ni1–xCoxFe2O4 tends to increase with decreasing cobalt ion content and with decreasing concentration of the point defects such as cation vacancies and Ni3+ ions.