In this study, we first investigated dispersion of the COOH functionalized carbon nanotubes (fCNT) in polyethylenimine (PEI) aqueous solution of pH 1-13 by solubility observation, zeta potential, and transmission electron microscopy. The fCNT/PEI dispersions of different pH were then cast on ITO electrode to investigate electrochemical behavior of the composites by cyclic voltammetry (CV) and electrochemical impedance spectroscopy. Solubility data found that the fCNT well dispersed in PEI aqueous solutions of pH 1-11 except 13 but precipitated in pure aqueous solutions of pH 1, 3, and 13. The fCNT dispersion improved by PEI in low pH solutions could be attributed to the interactions of the NH3+ of PEI and the COO- of fCNT. At a pH in the range of 3 to 11 at which ITO electrode was modified by the cast composite, the dispersed fCNT was found to electrocatalyze the redox reactions of the Fe(CN)(6)(3-/4-) probe and the PEI matrix could enhance the catalytic ability of fCNT leading to strong redox current intensity in CV curves. The fCNT/PEI composites cast from solutions at pH 1 and 13 exhibited no redox signals of the Fe(CN)(6)(3-/4-) probe. The pH variational fCNT dispersions in the fCNT/PEI composites and the interactions of Fe(CN)(6)(3-/4-) and NH3+ of PEI could be used to justify the CV curves. The fCNT/PEI composite cast from aqueous solution of pH 11 exhibited the least oxidation potential, the most reversible redox reactions, and the largest oxidation current intensity of the Fe(CN)(6)(3-/4-) probe. (C) 2011 Elsevier B.V. All rights reserved.