Using sol–gel auto-combustion method, monophasic spinel nickel ferrite Ni1−zFe2+zO4 (where z =0, 0.01; 0.025; 0.05; 0.075 and 0.10) gas sensors were prepared. The relationship between excessiron ion content and gas response was studied. Structural characterization was performed by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Increasing excess-iron ion content changes gas response behaviour of the nickel ferrite. Low excess-iron ion nickel ferrites (z <05) exhibit p-type conductivity, while compositions with high excess-iron ion content (z >05) n-type conductivity. Observed behaviour could be attributed to nickel ion substitution with iron ion in the octahedral site, thus giving possibility for electron hopping between Fe3+ ↔Fe2+ and n-type conductivity.