Fe- or Cu-containing Dawson-type tungtodiphosphates substituted by two, four or five Mo atoms are studied by cyclic voltammetry at pH 3 and 5. Their catalytic activities towards the reduction of nitrite are established and compared with those of the corresponding Fe- or Cu-substituted fully tungstic Dawson heteropolyanions. Whatever the pH, the presence of Mo atoms in the catalysts is found to be beneficial as concerns the current intensity or the potential location of the catalytic wave. Specifically, it is found that the first step with Fe derivatives is the formation of a nitrosyl complex which is further reduced at the potential of a subsequent wave of the heteropolyanion. In the case of Cu- and Mo-substituted compounds, the two heteroatoms are found to exert their beneficial influence in concert.