Platinum sols have been prepared by citrate reduction in the temperature range of 343-363 K. The Pt state in the solution was examined by EXAFS (extended X-ray absorption fine-structure spectroscopy). It did not show any Pt-Pt bonding, a characteristic for reduced Pt sols. EXAFS model fitting further proved the presence of Pt-O with 4 oxygen neighbors, which suggests a tetraplanar coordination configuration. The possibility of neighboring Pt sharing oxygen ligand or the formation of PtOx is rejected by EXAFS model fitting. Citrate was found to be the most likely ligand to orient its oxygen end toward a charged Pt center. Thus we have revealed that the citrate treatment at this temperature range was clearly insufficient to reduce H2PtCl6(aq). Neither an extended period of reaction time nor an excess citrate reduced the Pt precursor. It is therefore highly recommended that the citrate sols should be carefully prepared and used. (c) 2005 Elsevier Inc. All rights reserved.