The acid-base and electrochemical behaviour of the lacunary Dawson-type heteropolyanion alpha-[(1)-2,3-P2Mo2W15O61](10-) and of Its derivatives with V-IV, Mn-II, Fe-III, Co-II, Ni-II, Zn-II, and Cu-II has been studied as a function of pH. Essentially two groups can he distinguished. In the first group, constituted by the V-IV, Mn-II, Co-II, Ni-II, Zn-II-substituted complexes, the redox activity, if any, of the heterometallic cations occurs far from the potential location of that of the molybdenum moieties. At pH 3, the molybdenum waves are very close to each other and to that of the precursor lacunary complex. For higher pH values the apparent slowness observed for the first wave of the lacunary compound becomes less pronounced in the substituted complexes, but is modulated differently by each substituent. For the Fe-III and Cu-II substituted complexes, the redox activity of these cations can mix with that of the molybdenum centres, depending on the pH of the solution. Spectroelectrochemical experiments have helped to clarify the conditions of the separation of redox activities in the case of the Fe-III-substituted complex. It has been possible to obtain an overall three electron process on the first wave of this compound. In contrast, copper deposition is observed with the Cu-II substituted complex In appropriate conditions. All the complexes show electrocatalytic properties on their respective first wave for the reduction of NO. Owing to the very high stability of the V-IV-substituted complex throughout the whole pH domain, it has been possible to study the electrocatalytic oxidation of NADH to NAD(+) at pH 8.