The surface tensions of melts in the systems FeO–Fe₂O₃–CaO and FeO–Fe₂O₃–2CaO·SiO₂ have been measured under air and a CO₂ atmosphere from 1673 to 1913 K using the maximum bubble pressure method. The temperature dependence of the surface tension in both systems is very low. The surface tension of iron oxide melt under air is 605±7 mN/m at 1873 K which suggests that the ferric ion in iron oxide melt is not surface active. The addition of CaO to the melt decreases the surface tension. The addition of 2CaO·SiO₂ causes an appreciable lowering of the surface tension, but an anomalous arrest is observed when the melt contains 5.3–11.1 mol% 2CaO·SiO₂. This is presumably caused by the appearance of a liquid miscibility gap in the system.