We have used the density functional B3LYP method to study the effect of hydrogen bonds from the histidine ligand in various haem proteins to carboxyl groups or to the carbonyl backbone. Hydrogen bonds to carbonyl groups (encountered in globins and cytochromes, for example) have a small influence on the geometry and properties of the haem site. However, hydrogen bonds to a carboxyl group (encountered in peroxidases and haem oxidase) may have a profound effect. The results indicate that in the Fe3+ state, this leads to a deprotonation of the histidine ligand, whereas in the Fe2+ state, the proton involved in the hydrogen bond may reside on either histidine or the carboxylate group, depending on the detailed structure of the surroundings. If the histidine is deprotonated, the axial Fe-N bond length decreases by 0.15 Å, whereas the equatorial bond lengths increase. Moreover, the charge on iron and histidine is reduced, as is the spin density on iron. Most importantly, the energy difference between the high and intermediate spin states changes so that whereas the two spin states are degenerate in the Fe2+ state for the protonated histidine, they are degenerate for the Fe3+ state when it is deprotonated. This may facilitate the spin-forbidden binding of dioxygen and peroxide substrates, which takes place for the Fe2+ state in globins but in the Fe3+ state in peroxidases. The reduction potential of the haem group decreases when it hydrogen-bonds to a negatively charged group. The inner-sphere reorganization energy of the Fe2+/Fe3+ transition in a five-coordinate haem complex is ˜30 kJ mol−1, except when the histidine ligand is deprotonated without any hydrogen-bond interaction.