The effects of adding iron to Pd-Pt/sulfated zirconia (SZ) on the selective NOx reduction by methane were examined based on durability tests under conditions simulating natural gas combustion exhaust. While Pd-Pt/SZ was severely deactivated at 500 degreesC, Pd-Pt/Fe-SZ maintained a NOx conversion higher than 70% for over 2400 h under the same conditions. Methane conversion on Pd-Pt/Fe-SZ was significantly lower than that on Pd-Pt/SZ. XRD analysis of fresh and used catalysts showed that a part of the SZ had transformed to monoclinic ZrO2 and that adding Fe suppressed the transformation. These results suggested that the improvement in NOx conversion by adding Fe was due to the suppression of methane combustion and the stabilization of SZ against transformation to ZrO2.