It is demonstrated for the first time that the gas sensing properties of a spinel ferrite complex metal oxide semiconductor can be improved by controlling iron stoichiometry. Conductivity and sensitivity was analysed for ZnFe2+zO4± spinel type ferrite with z from -0,01 to 0,15. By increasing iron content from z=-0,01 to z=0,1, sensitivity increases up to 3 times but for samples with z>0,1 sensitivity drops. It was observed from impedance spectroscopy measurements that resistance decreases with an increase of the iron content. Complex impedance spectra reveal two phases with different resistance attributed to depletion layer (Rd) and bulk (Rb). With increasing iron content increases Rd/Rb ratio due to decrease of depletion layer width. Outer surface of the grain of the sensing material estimated higher ratio (Rd(air)/Rd(gas)) with gas exposure in comparison with bulk (Rb(air)/Rb(gas)). Overall sensitivity increases due to increase of donor concentration leading down to increase adsorbed oxygen species on grain surface. Sensitivity drops when z > 0,1 due to remarkable decrease of depletion layer width.