Activities of CrO and CrO_1.5 were determined in slags based on the systems of CaO-SiO₂-CrO_x, CaO-SiO₂-MgO-CrO_x, CaO-SiO₂-Al₂O₃-CrO_x, and CaO-SiO₂-MgO-Al₂O₃-CrO_x with electro-motive force method. Equilibrium was achieved between Cr-saturated CrAg alloy and slags. Effects of temperature and slag composition on the activities of chromium oxides and the oxidation state of chromium in the slags were investigated. Chromium oxides activities decreased but the divalent chromium fraction in the slags increased when elevating temperature in the range of 1500-1600°C. The slag basicity has a significant effect, the higher the slag basicity, the higher the activities of chromium oxides, and the lower the divalent chromium fraction. By partial substituting MgO for CaO, activities of CrO and CrO_1.5 will decrease, but the effect on the oxidation state of chromium in the slag is not obvious. Increasing Al₂O₃ content from 0 to 10 mol% results in higher activities of chromium oxides and lower divalent chromium fraction. Further increasing of Al₂O₃ content to 20 mol% does not cause any significant effect. In addition, activities of chromium oxides and oxidation state of chromium in a FeCr type slag system were examined.