Reductive titrations of the dissimilatory hexa-haem nitrite reductase, Wolinella succinogenes, with methyl viologen semiquinone (MV) and sodium dithionite, have been followed at room temperature by absorption, natural (CD) and magnetic circular dichroism (MCD) spectroscopies and at liquid helium temperature by electron paramagnetic resonance (EPR) and MCD spectroscopies. The nature of the reduced enzyme depends on the reductant employed. At room temperature a single high-spin ferrous haem, observed by MCD after reduction with MV, is absent from dithionite reduced samples. It is suggested that a product of dithionite oxidation becomes bound with high affinity to the reduced state of the enzyme causing the ferrous haem to become low-spin. The site occupied is likely to be the substrate binding haem. The course of the titration with MV at room temperature shows the reduction of high-spin ferric to high-spin ferrous haem. Since the EPR spectrum reveals the presence of an unusual high-low spin ferric haem pair in the oxidised state we propose that the active site of the enzyme is a novel haem pair consisting of one high (5-coordinate) and one low-spin (6 coordinate) haem, magnetically coupled and possibly bridged by a histidinate ligand.