High-resolution transmission electron microscopy (HRTEM), microchemical analysis by X-ray emission spectroscopy (EDS), and X-ray diffraction (XRD) were used to characterize the composition, size, distribution, and morphology of the phases present in three different carbon-supported Pt-Sn catalysts. The catalysts had nominal Pt/Sn atomic ratios of 1: 1 (500N and 900N samples) and 3:1 (E270 sample). The 1: 1 catalyst sample heat-treated at 500 ° C (500N) contained a Pt-rich fcc alloy phase (a(0) = 0.3965 nm) and tetragonal SnO2. The sample from the same precursors heat treated at 900 ° C (900N) consisted of stoichiometric hexagonal PtSn, a nearly stoichiometric Pt3Sn fcc phase (a(0) = 0.3988 nm), and tetragonal SnO2. Neither pure tin nor pure platinum particles were detected in any of the catalysts. The 3 : 1 catalyst sample reduced in H-2 at 270 ° C (E270) was composed entirely of the stoichiometric Pt3Sn cubic phase (a(0) = 0.3998 nm). The Pt3Sn-type particles are predominantly single nanocrystals with mean diameters of ∼ 4 nm (for the catalysts E270 and 500N) and ∼ 5 nm (for the catalyst 900N), whereas the PtSn particles were larger. The Pt3Sn-type particles in 500N and 900N were generally highly faceted, with {111} and {200} facets, whereas the E270 catalyst had generally a smooth spheroidal outline. Whereas the Pt3Sn particles in the E270 and 900N catalysts were equiaxed, in the 500N catalyst the alloy particles had uniquely elongated (e.g., ellipsoidal) shapes. © 2005 Elsevier Inc. All rights reserved.