To probe distortions of the haem group in myoglobin and haemoglobin resulting from ligand—haem and protein—haem interactions, the dispersion of the depolarization ratio and the resonance excitation profiles of the Raman line ν4 were analysed in terms of parameters describing intra- and intermanifold vibronic coupling between the Q and B states of the porphyrin macrocycle. These parameters depend linearly on symmetry classified normal distortions of the haem group. It is shown by group theoretical arguments that perturbations at the pyrrole nitrogens N(I) and N(III) of the haem cause B1g-type distortions which increase with increasing tilt and bend angles of the proximal imidazole and the distal O2 ligand, respectively. Moreover, antisymmetric A2g-type distortions are imposed via perturbations at the Cα atoms of pyrrole I and III when one of the ligands moves away from a position in which it eclipses the N(I)[BOND]Fe2+[BOND]N(III) line of the porphyrin. It is found that the influence of the proximal and distal ligand on the haem symmetry is different in myoglobin and haemoglobin. In oxymyoglobin the O2—haem interaction is responsible for the comparatively large B1g distortion at N(III). B1g distortions in oxyhaemoglobin and its isolated subunits are smaller then in oxymyoglobin. They are mainly caused by repulsive interactions between haem and proximal imidazole. A2g distortions at Cα(I) and Cα(III) are negligible in oxymyoglobin and small in deoxymyoglobin and deoxyhaemoglobin. They are comparatively strong, however, in oxyhaemoglobin and its isolated subunits, owing to the proximity and the large azimuthal angle of the proximal imidazole. In the isolated β-chains of oxyhaemoglobin haem—protein coupling is more effective and causes larger B1g and A2g distortions than in the α-chains. While the haem distortions investigated are independent of pH in the isolated α-chain, they depend significantly on pH in the isolated β-chain. This is caused by protonation of three titrable groups with pK = 5.8, 6.6 and 7.8, which were earlier assigned to His Eβ, His HC3β and His FG4β, respectively. In intact oxyHbA the allosteric coupling between these titrable groups and haem are modified by strengthening that of His HC3β and reducing that of His FG4β. Coupling between the former group and the haem is probably caused by the hydrogen bond between the penultimate Tyr HC2β and Val FG5β and steric interactions between Tyr HC2β and Cys F9β. Consequently, it is completely absent in the modified oxyhaemoglobin bis(maleimidomethyl) ether, in which the hydrogen bond between the penultimate Tyr HC2β and Val FG5β is ruptured.