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A systematic reexamination of the Er - Co system was carried out by differential thermal analysis, optical microscopy, X-ray diffraction and EPMA techniques. A revised Er - Co binary phase diagram is proposed. This binary system consists of four peritectic reactions, three reactions of congruent melting and a eutectoidal reaction. The four peritectic reactions result in the formation of Er3Co, ErCo2, Er2Co7 and ErCo5, respectively. The three congruent reactions involve the formation of almost-equal-to Er16Co9, ErCo3 and Er2Co17. The eutectoidal reaction involves the decomposition of ErCo5 into Er2Co7 and Er2Co17. There are also four eutectic reactions in this system. The solid solubility of Er in Co is extremely small.