Protein film voltammetry has been used to define the catalytic performance of two nitrate reductases: the respiratory nitrate reductase, NarGH, from Paracoccus pantotrophus and the assimilatory nitrate reductase, NarB, from Synechococcus sp. PCC 7942. NarGH and NarB present distinct ‘fingerprints’ of catalytic activity when viewed in this way. Potentials that provide insufficient driving force for significant rates of nitrate reduction by NarB result in appreciable rates of nitrate reduction by NarGH. However, both enzymes display complex modulations in their rate of substrate reduction when viewed across the electrochemical potential domain.