Knowledge of the exact nature of the iron–oxygen bond in oxyhaemoglobin (oxyHb) is essential for the understanding of cooperative oxygen binding to haemoglobin (Hb)¹. However, X-ray studies of oxyHb have previously been hindered by the tendency of oxyHb crystals to autoxidize. Here we report the stereochemistry of the haem–oxygen complex in human oxyHb, as determined by single-crystal X-ray analysis. Bent end-on geometry of the iron–oxygen bond in haem proteins, as predicted by Pauling², was first observed in the ‘picket fence’ complex³ and has since been observed in oxymyoglobin (oxyMb)^4,5; in oxyerythrocruorin, however, the Fe—O—O bond is almost linear⁶. In oxyHb the Fe—O—O bond angle is 156°, intermediate between the ‘picket fence’ compound and erythrocruorin. The position of N^ɛ of His E7 in the α-subunit suggests that it forms a hydrogen bond with the bound oxygen, as in oxyMb⁷. In the β-subunit, however, N^ɛ of His E7 is located further from the oxygen, suggesting that the hydrogen bond is weaker.