Gold catalysts for low-temperature CO oxidation were prepared by immobilizing gold colloids of about 2 nm size on TiO2 and ZrO2 in aqueous solution. The gold particles nearly retained their size after immobilization on both supports. Slight sintering was observed by X-ray diffraction and high-resolution transmission electron microscopy after calcination at 673 K. Some of the gold particles on ZrO2 showed hexagonal symmetry with close-packed Au(111) surfaces. X-ray photoelectron spectroscopy investigations indicated that the gold was in a metallic state. Whereas gold particles on TiO2 showed CO conversion directly after preparation and drying, significant activity of gold on ZrO2 was only observed after calcination in air. CO adsorption was reversible on all catalysts and weaker on the most active catalysts, as shown by diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and pulse thermal analysis. Probing of the gold sites by CO adsorption revealed the existence of different gold sites in these catalysts. Step/kink sites (2111-2123 cm(-1)) and positively polarized gold sites (2128-2135 cm(-1)) were identified on uncalcined and in situ calcined gold catalysts. The number of low-coordinated gold sites was much higher on TiO2, whereas on ZrO2 more positively polarized gold atoms were found. This behaviour was traced to the shape of the gold particles, which affects the number of low-coordinated gold atoms and is dependent on support and treatment, in situ treatment in oxygen led to a temporarily inactive catalyst with additional bands in the DRIFTS spectrum in the 2000-2400 cm(-1) region, stemming from CO adsorbed on positively polarized gold sites (2130-2150 cm(-1)) and on the support (2180-2200 cm(-1)), and from molecularly adsorbed CO2 (2353 cm(-1)). No bands arising from CO on low-coordinated, metallic gold sites were identified in that case,