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The influence of partial replacing of Mg and Fe ions by Sn and Mo ions, respectively, on the structural and conductivity properties of MgFe2O4 ferrite is studied by means XRD, SEM and resistivity measurements. Four compositions of ferrites, Mg1–xSnxMoyFe2–yO4 (x = 0, 0.1 and y = 0, 0.02) were prepared by autocombustion, starting from metal nitrates. The presence of foreign phase was evidenced in the Sn containing ferrite. Also, Sn and Mo ions induced structural changes regarding grain size and porosity. The finest granulation, of about 100 nm, was observed in the Sn containing sample, Mg0.9Sn0.1Fe2O4 having the highest porosity. The electrical resistivity increased with about two orders in magnitude when Sn4+ ions partially substituted Mg in MgFe2O4 ferrite. All samples were tested for sensing properties to reducing gases: ethanol and acetone. The gas sensitivity considerably depends on the kind of substituent ion and the type of gas to be detected. All the ferrites were more sensitive to acetone than to ethanol. It was revealed that among all ferrites tested, Mg0.9Sn0.1Fe2O4 is the most sensitive to acetone. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)