AbstractEquilibrium segregation of phosphorus at grain boundaries of austenite was studied in Fe–P and Fe–10Mn–P (wt-%) alloys. The specimens were equilibrated at temperatures between 750 and 1200°C and after rapid quenching analysed using Auger electron spectroscopy. It was found that phosphorus segregated in both types of alloy and it is shown by the results that manganese increased the level of phosphorus segregation. There was no observation of segregation of manganese in austenite. Segregation of phosphorus in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. The equilibrium segregation of phosphorus in the Fe–P system and also in the Fe–10Mn–P system can be described using the Langmuir–McLean equation and a value for the free energy of phosphorus segregation∆GPo= 47 kJ mol−1. It is most probable that the level of phosphorus segregation is enhanced by manganese because manganese intensifies the additional segregation of phosphorus during quenching.MST/876