It was generally accepted that the total elimination of chlorine is favorable to build active supported nano-gold catalysts if chlorine-containing precursors was used especially for the total oxidation of CO. Here a series of Au/NiOx catalysts were prepared with co-precipitation method by tracing the chlorine concentration in the washing step and tested by the total oxidation of CO. It was revealed that Au/NiOx catalysts prepared from aqueous solutions with 4-8 ppm chlorine can be active catalysts and the temperature for CO total oxidation was <0 degrees C. The catalytic activity dropped sharply if the chlorine concentration was <4 ppm or >8 ppm. The catalysts were characterized by XRD, XPS, BET, TEM, HAADF-STEM and EXAFS, and the catalyst structures were explored. The different structures of the catalysts should be responsible for the activity variation and it was caused by the washing operation or the presence of different amounts of chlorine. Although active Au/NiOx catalysts can be obtained by tracing chlorine concentration, the role of chlorine in the fabrication of active nano-gold catalyst was still unclear. (C) 2013 Elsevier B.V. All rights reserved.