The equilibrium distribution of potassium between liquid Mn-Fe-C alloys and molten K2O-containing MnO-SiO2-CaO slags in CO atmosphere were studied experimentally at 1400-1500oC. Experimental results indicate that potassium preferably goes to the gas phase under reducing conditions. The solubility of K in Mn-Fe-C alloys is negligible. The K2O content in all the slag samples is lower than those in industrial slags. Carbon plays an important role in the K-distribution equilibria. Potassium oxide is probably reduced to gaseous potassium rather than to metallic potassium in the metal phase. Based on the equilibrium measurements, thermochemical behavior of K in Mn-based alloys was evaluated. The results of the model calculations have been used to explain the experimental observations.