The size effect of monometallic Pd and Pt clusters was investigated for the sulfur tolerance of these noble metal catalysts supported on high-silica USY zeolite, using the CO adsorption method and the extended X-ray adsorption fine structure (EXAFS) method. Both Pd and Pt particles show the highest surface sulfur tolerance in the form of < 20 angstrom-diameter particles, although the bulk phase of these clusters is subject to penetration by adsorbed sulfur atoms. For noble metal particles > 25 angstrom in diameter, the bulk phases of the Pd and the Pt particles approach that of the metallic structure. With increased cluster size, the surface sulfur tolerance of Pt particles decreases gradually, while that of Pd particles decreases rapidly. For large noble metal particles > 50 angstrom in diameter, EXAFS spectra show that sulfidation in the bulk phase of the noble metal particles is suppressed for Pt but promoted for Pd. (c) 2005 Elsevier B.V. All rights reserved.