A sulfur-tolerant Pt catalyst has been developed for fuel processors being developed for use with fuel cells, using a fluorite-type Ce0.8Gd0.2O1.9 support. The catalyst calcination temperature is crucial to ensure the maintenance of sulfur tolerance. The catalyst calcined at 800 ◦C retained its activity and selectivity for entire 100-h test period in the steam reforming of iso-octane with 300 μg/g of sulfur, whereas the catalyst calcined at 600 ◦C obviously lost activity in this course. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for CO and CO/H2S adsorption was performed to characterize the nature of the Pt sites and to evaluate their ability to tolerate sulfur. Strongly electron- eficient Pt sites, evidenced by a CO adsorption band at νmax 2104-2108 cm−1, were immune to sulfur poisoning and were uniformly formed with the alcination at 800◦C. In addition, thiophene sulfur was completely converted into H2S and likely complied with a redox mechanism.