The results of surface tension measurements for melts of the Fe-Mn-C system with 5.0-25.0 wt % Mn and 0.4-2.2 wt % C were given. Anomalies in the temperature dependences of the surface tension coefficient of Fe-Mn-C melts were found, which are interpreted as evidence for the irreversible destruction of microheterogeneity. The microheterogeneity destruction temperature was determined.