3, 5, and 12-nm Co nanoparticles were synthesized via thermo-decomposition. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) were used to probe the structures of these nanoparticles. Simulations were carried out to compare the EXAFS profiles of three different crystalline phases of Co. The results of EXAFS and XANES suggested that the reactivity of these nanoparticles towards their environment is inversely proportional to their size. Sharp decreases in the amplitude of EXAFS of these nanoparticles were observed. which could be attributed to the existence of the disorder in those crystalline nanostructures. (C) 2004 Elsevier B.V. All rights reserved.