To develop HCHO oxidation catalysts which work at moderate temperatures, a series of Au/CeO2 catalysts with a gold content below 0.85 wt.% were prepared by co-precipitation and subsequent calcinations at 300 degrees C. Oxidation of formaldehyde on these catalysts at temperatures close to 100 degrees C was conducted, and the structures of catalysts were characterized by X-ray diffraction (XRD) and transmission electronic microscopy (TEM) techniques. Gold exists in highly dispersed crystallite clusters in these catalysts, and we did not observe any gold crystals larger than 23 nm. In contrast, the CeO2 support is well crystallized, the fringes from (1 1 1) lattice plane of the support CeO2 are very clear in the TEM image. Gold crystals with a mean size of about 10 nm formed in a sample containing 0.78 wt.% of gold (0.78Au) when it was calcinated at 400 degrees C for 2 h. However, the formation of larger gold crystals causes a decrease in the catalytic activity. It appears that the highly dispersed gold catalyst provides more active sites for the HCHO oxidation. When the sample 0.78Au was calcined at 700 degrees C for 2 It, large gold particles (>= 50 nm) appeared and the activity for HCHO oxidation decreased further but was still better than that of ceria. XRD and XPS results show an interesting fact that some of gold was incorporated into lattice of ceria. (c) 2007 Elsevier B.V. All rights reserved.