The effects of sulfur poisoning on the structural changes of gamma-alumina-supported Pt catalysts were investigated by X-ray absorption spectroscopy and temperature-programmed reduction (TPR). Extended X-ray absorption fine structure (EXAFS) analysis of the fresh catalyst determined an average Pt-Pt coordination number of 2.6, suggesting an average platinum cluster size of 3 and 4 atoms formed on the gamma-alumina surface. After the test reaction using a tetralin feed containing 1000 ppm sulfur at 270 degrees C and 32 atm for 24 h, an increase of Pt cluster size was observed. A concomitant increase of the Pt-Pt coordination number to 7.3 with the formation of Pt-S contribution in EXAFS indicate that metal migration was induced by sulfur poisoning. The Pt-S contributions with bond distance of 2.34 and 2.53 Angstrom were obtained by subtracting the Pt-Pt contribution from Fourier-isolated raw data. The results, combined with TPR peaks characterizing the sulfur-poisoned catalysts, suggest that the two Pt-S contributions were due to the interaction between Pt and the adsorbed H2S and the formation of platinum sulfide (PtS). After the fresh catalysts were tested with more severe operation conditions (19 atm, 2000 ppm sulfur), the used catalysts were characterized by only one Pt-S contribution of bond distance 2.31 Angstrom, indicating that clusters with a morphology closely resembling bulk PtS were formed on the surface. Based on the structural investigation results, we conclude that the loss of catalyst activity during sulfur poisoning is attributed to the adsorption of H2S, sulfur-poisoning-induced Pt agglomeration and the formation of PtS.