A simple electrochemical silicon sensor was fabricated using a solid MgO-stabilized ZrO₂ electrolyte oxygen sensor with ZrO₂+ZrSiO₄ auxilliary electrode. Silicon activities in liquid Fe-C-Si and Fe-B-Si alloys were measured using the sensor at 1 773 and 1 715 K, respectively. Reliable measurements were preformed in Fe-C-Si melts at up to 3 wt% Si and in Fe-B-Si melts at up to 10 wt% Si. Results were in good agreement with calculated values from melt compositions. The value of 0.23 was proposed for the interaction coefficient, e_Si^B, at 1 715 K. The silicon sensor was proved to be applicable for the in situ determination of silicon activities or concentrations in Fe-C-Si and Fe-B-Si melts.