The electrochemical reduction process of 1,10-phenanthroline-5,6-dione (pdon) is chemically irreversible in pure aqueous solution at high pH values (pH >3.6) due to the formation of insoluble reduzate, 1,10-phenanthroline-5,6-diol (pdol), which gets away from the electrode surface and avoids being re-oxidized. This issue may be circumvented by adsorption of pdon on carbon nanotubes (CNTs) electrode. The adsorbed redox couple (pdon/pdol) well keeps its reversible electrochemical behavior even the pH value is expanded to 9.0. Thereby, it is possible to investigate the redox reaction mechanism of pdon/pdol in not only acidic but neutral and alkaline media as well. Interestingly, the reduction of pdon undergoes three distinct reaction paths and gives rise to variant reductive forms at different pH regions. The absorbed pdon also exhibits excellent electrocatalytic activity towards the oxidation of ascorbic acid (AA). The over potential is significantly lowered, which makes it possible to selective measurement of AA in the presence of dopamine (DA) and uric acid (UA). (C) 2012 Elsevier B.V. All rights reserved.