It is shown that with respect to the oxygen solubility of liquid silicon, an electrochemical oxygen sensor based on a solid-oxide electrolyte is not qualified to provide better solubility data than the analyzing techniques used to date. The crucial obstacle is that under the operation conditions the solid electrolyte exhibits a remarkable electronic conduction which substantially falsifies the evaluation of the experimental data. This is revealed by a strict thermodynamic approach to the essential points of the experimental procedure which results in the determination of the oxygen solubility.