Hydrogen cyanide (HCN) aqueous solutions can remove copper (Cu) contaminants from SiO2 surfaces completely even when the cleaning is performed using dilute (e.g., 0.027 wt %) HCN aqueous solutions at 25 degrees C. When pH of the HCN solutions is set at 10, Cu contaminants with a concentration of similar to 2 x 10(12) atoms/cm(2) can be removed below similar to 3 x 10(9) atoms/cm(2) taking only 10 s. The concentrations of Cu+ and [Cu(CN)(2)](-) ions in the HCN solutions drastically decrease with pH, while those of [Cu(CN)(3)](2-) and [Cu(CN)(4)](3-) ions increase. The dissociation probability of HCN exponentially increases with pH. Therefore, the improvement of the cleaning ability with pH is attributable to (i) an increase in the concentration of CN- ions, and (ii) the prevention of readsorption due to the formation of stable Cu - cyano complex ions. Measurements of surface Cu concentrations on the SiO2 surfaces contaminated by the immersion in the CuCl2-containing HCN solutions clarify the mechanism of Cu removal. The rate-determining step is a reaction between SiO-CuCN on the surface and CN- ions in the solution to form [Cu(CN)(2)](-) complex ions. The equilibrium constant of this reaction is found to be 4.1 x 10(5) M. This large value shows that desorption of [Cu(CN)(2)](-) proceeds much more easily than adsorption. (c) 2006 The Electrochemical Society. [DOI: 10.1149/1.2178649] All rights reserved.