A cyclic voltammetry study on the anodic reaction between carbon electrodes and CaO-SiO2-Al2O3 melts at 1773 K was carried out to understand the electrochemical behavior of free oxygen ions in silicate melts. The potential of free oxygen ions corresponding to CO evolution in the totally diffusion controlled region was obtained from the voltammetric data. An empirical relationship between the current density on the logarithm scale and the basicity (B) of each silicate melt was established through Tafel analysis in the non-diffusion-controlled region. The obtained electrochemical data showed good correlation with thermodynamic properties such optical basicity and sulfide capacity of the silicate melts as a relative index of oxygen ion activity in molten oxides. The use of this electrochemical method to evaluate the relative potential of oxygen ions in molten oxides is discussed in detail. (C) The Author(s) 2017. Published by ECS. All rights reserved.