The electrochemical quartz crystal microbalance (EQCM) based on the piezoelectric quartz crystal impedance (PQCI) analysis was used to investigate the electrolyte effects on the precipitation of phenazinehydrine charge-transfer complex (CTC) during cyclic voltammetric redox switching of o-phenylenediamine (oPD) at Au electrodes in aqueous H2SO4. The response to concentration modulation for Na2SO4, K2SO4, sodium gluconate, tannic acid (TA), or sodium heparin as additives in 0.10 (or 0.30) mol L-1 aqueous H2SO4 containing 0.050 mol L-1 o-phenylenediamine was comparatively examined. In vivid contrast to SO42- and gluconate, the increase in the concentration of large-sized TA or sodium heparin strongly weakened the CTC precipitation, giving detectable responses down to the mu mol L-1 level. Similar effects were also found in acidic phenazine systems. The anionic effect on the CTC precipitation is reasonably explained via the following mechanism, namely, the protonation at nitrogen atoms of the oligomers with phenazine units from oPD oxidation, induced anionic incorporation and resulting steric hindrance in CTC-formation reactions/processes, plus the concentration decrease for oligomers with phenazine units in the presence of large-sized anions. (c) 2006 Elsevier B.V. All rights reserved.