Temperature-programmed reduction (TPR) and temperature programmed desorption (TPD) of NH3 were used to characterize bimetallic interactions and the acidity of mordenite-supported Pt and Ni-Pt catalysts. The effects of adding Ni to the Pt catalysts were investigated by examining (1) total conversion, fuel gas formation, and rate of branched isomer formation of pure C-5 (n-pentane), C-6 (n-hexane), and C-7 (n-hexane) reactions catalyzed by the Pt and Ni-Pt catalysts; and (2) the stability of the catalysts with C-6 feed containing 500 ppm sulfur. The test reactions were carried out under operating conditions similar to those of a commercial isomerization process. For pure feed, the reaction results indicated that addition of a moderate amount of Ni to the Pt catalyst not only suppresses fuel gas formation, but facilitates the formation of branched isomers. However, for sulfur-containing feed, no suppression of fuel gas formation was observed. Instead, the Ni-Pt catalyst underwent a more rapid deactivation and produced more fuel gas than did the Pt catalyst. Together with the catalytic performance test and the TPD and TPR results, the relatively high catalyst deactivation and fuel gas formation rate are associated with the poor sulfur resistance of the catalyst caused by Ni-Pt bimetallic interactions.