This study aimed to determine the influence of Ir content on the behavior of Pt-Ir/C catalysts for hydrogen iodide decomposition in iodine-sulfur thermochemical water-splitting cycle. A series of bimetallic xPt-yIr/C catalysts (x = 4, 3, 2.5, 2, 1 wt.%; y = 1, 2, 2.5, 3, 4 wt.%) were prepared by co-impregnation and reduction method. For comparison, monometallic 5%Pt/C and 5%Ir/C catalysts were also prepared. The catalysts were characterized by X-ray diffraction and transmission electron microscopy. Changing the Ir content in the bimetallic Pt-Ir/C catalysts can modify the particle size and dispersion state of the supported metal nanoparticles. The results of the characterizations of the fresh and used catalysts indicated that the size of the Pt nanoparticles has significantly increased after HI decomposition whereas that of the Pt-Ir nanopartides is more resistant to particle growth. Among the Pt, Ir, and binary Pt-Ir catalysts, the bimetallic 2.5%Pt-2.5%Ir/C exhibited the highest catalytic activity toward HI decomposition. The 70 h durability test showed that the Pt-Ir bimetallic catalyst possessed good activity maintenance. (C) 2014 Elsevier B.V. All rights reserved.