A tetraoctylammonium-stabilized PtSn colloid with nominal composition Pt3Sn has been prepared by coreduction of the metal salts. This colloid, which is used in the manufacture of anode catalysts for low-temperature polymer membrane fuel cells, was supported on silica and the structure studied by in situ X-ray diffraction and Debye function analysis. Results indicate that the supported colloid is best described as a highly disordered bimetallic fee cluster with a particle size of similar to 1.3 nm. From TEM images a slightly larger size of 1.5 +/- 4 nm is derived. Point-resolved EDX (energy-dispersive X-ray spectroscopy) confirms the elemental ratio of 3:1 in the PtSn particles. However, the XRD simulation indicates some deviations from uniform alloy formation. On removal of the stabilizing tetraoctylammonium ligands by heating the samples in He to similar to 200 degrees C, coalescence of neighboring particles occurs. The new alloy formed consists of a majority of cubic Pt0.81Sn0.19 particles, with average size 3.1 nn, and a minority phase of stoichiometric hexagonal PtSn, 4.4 nm in size. The former phase can be considered as a metastable supersaturated solid solution of Sn in Pt. The two phases are stable even on heating to 375 degrees C for 1.5 h in He.