The selective oxidation of CO in a stream rich in H-2 was investigated over a series of Au/gamma-Al2O3 catalysts. It was found that the presence of MZ: citrate in the preparation solution had a significant effect on the properties of the catalyst. Catalysts prepared without citrate contained larger Au particles, were less active catalytically, and were less selective for CO oxidation. Although the experimental uncertainties were large, there appeared to be an optimal average Au particle size of about 5-10 nm for best activity and selectivity. At 100 degrees C, about 50% selectivity was obtained over the more selective catalysts, which depended only weakly on either O-2 or CO conversion.