A Mo(V) electron paramagnetic resonance (EPR) study of the periplasmic respiratory nitrate reductase of the denitrifying bacterium Thiosphaera pantotropha has revealed that the molybdenum centre of this enzyme is very similar to that in the assimilatory nitrate reductase of Azotobacter vinelandii but is somewhat different from that of the membrane‐bound bacterial respiratory nitrate reductases such as those of Escherichia coli and Paracoccus denitrifcans. We have identified the Mo(V) species most likely to be the catalytically relevant one and characterised two other sets of Mo(V) EPR signals. As well as exhibiting EPR signals with g values typical of bacterial molybdenum‐containing reductases, molybdenum‐hydroxylase‐like EPR signals can be elicited in the nitrate reductase of T. pantotropha upon treatment with excess dithionite. The only other enzyme known to display this phenomenon is the periplasmic dimethylsulphoxide reductase of Rhodobacter capsulatus. A mechanism for the generation of these signals is proposed which invokes reduction of the pterin ring of the molybdenum cofactor linked to GMP from the dihydro to the tetrahydro state. The possibilities and implications of there being cysteine ligands to the molybdenum centres of these two enzymes are discussed.