Recent results of directional solidification experiments with Fe-Co alloys were not consistent with the presently accepted peritectic phase diagram. Therefore, the Fe-rich part of this phase diagram was re-examined. High-temperature gradient plane front solidification was carried out at a low velocity. After quenching of the specimen from the peritectic temperature, the compositions of the phases close to the solid/liquid interface were determined through electron probe microanalysis. Using these results together with previously published phase diagram information, an improved phase diagram is proposed with the peritectic equilibrium at T-p = 1774 K, C-delta = 17.7, C-gamma = 18.1 and C-L = 19.5 at.% Co.