Redox ratios of chromium and antimony were directly measured at 1000°C in sodium silicate melts by electrochemical methods. The redox ratios were found to increase while increasing the sodium oxide content, i.e. the liquid basicity. It was shown that the logarithm of the redox ratio is a linear function of the logarithm of the sodium oxide activity. Our results differ systematically from those obtained on quenched samples. The results are discussed in the light of a thermodynamic model considering activities of molecular species. We suggest that, when dissolved in silicate melts, multivalent ions form oxocomplexes which are not fully discrete species because non-bridging oxygens can intervene in their coordination sphere. We also concluded that there are rapid exchanges of oxygen atoms between the oxocomplexes and the solvent.