We have studied the effect of modifying Pt/gamma-Al2O3 catalysts with magnesium on the oxidation of methanol and carbon monoxide in presence of ammonia. The purpose for adding magnesium to the catalysts was to enhance the low-temperature activity and to make catalysts more resistant towards ammonia inhibition. The catalysts were prepared by wet-impregnation using platinum nitrate and magnesium nitrate solutions. The platinum content was constant for all catalysts, while the magnesium content ranged from 0 to 5 wt.%. The catalysts were characterized by CO chemisorption, AAS, XRD, XPS and BET. The addition of magnesium resulted in the formation of both MgAl2O4 spinel and MgO, and did slightly improve the low-temperature activity for carbon monoxide oxidation, while the activity for methanol oxidation decreased compared to an unpromoted Pt/gamma-Al2O3 catalyst. The inhibition by ammonia for carbon monoxide oxidation was low for all catalysts, while the methanol oxidation was substantially inhibited by ammonia. Adding magnesium to Pt/gamma-Al2O3 catalysts did not make the catalysts more resistant to ammonia inhibition. (C) 2004 Elsevier B.V. All rights reserved.