The effect of mixed sulfur oxides pretreatment on the surface state and carbon monoxide oxidation activity of CuxZn1-xCr2O4 mixed oxides, has been investigated using X-ray photoelectron spectroscopy (XPS) and kinetic measurements. The results obtained indicate that SO(x) species bind preferentially to the surface zinc ions, which thus protect from poisoning the surface copper ions which are the active species for carbon monoxide oxidation.