At low pH, trivacant polyoxometalate alpha-[P2W15O56](12-) reacts with titanyl and forms tetrameric {K28H8[P2W15Ti3O60.5](4)} (1). The purity of [P2W15Ti3O60.5](4)(36-) has been confirmed by IR spectroscopy, elemental analysis, and P-31 NMR spectrometry. The electrochemical behaviour of the tetrameric complexes in aqueous solution has been investigated at various pHs. [P2W15Ti3O60.5](4)(36-) exhibits successive reduction processes of the W atoms in a negative potential range. All waves are pH dependent at pH < 4. The anion showed an electrocatalytic effect on the reduction of HNO2,, but this effect is greatly influenced by pH. At low pH (pH < 3.5) the cathodic peak current of the first W-VI/V couple is enhanced while its anodic couterpart disappears. When pH > 3.5 no catalytic effect is observed. On the contrary, upon saturation of the POM solution with NO, even at pH > 3.5, electrocatalytic reduction of NO was observed. The tetrameric tri-Ti-IV-substituted Wells-Dawson polyoxometalate [P2W15Ti3O60.5](4)(36-) was not degraded by repeated cycling between its oxidized and reduced states. (C) 2004 Elsevier B.V. All rights reserved.