The adsorption of cetyltrimethylammonium bromide (CTAB) on a carbon paste electrode was investigated by voltammetry and electrochemical impedance spectroscopy. The results showed that CTAB exhibited two types of adsorptive behavior at a carbon paste electrode at different CTAB concentration ranges: monomer adsorption at concentrations below 3.0 x 10(-6) M and monolayer adsorption at concentrations higher than 3.0 X 10-6 M. In the monomer adsorption range, the adsorption of CTAB could effectively reduce the charge transfer resistance and increase the charge transfer rate. However, the surface properties of the carbon paste electrode hardly changed. On the formation of a CTAB monolayer, the structure of the electrode interface changed significantly and the electrode\solution interface was replaced by the CTAB monolayer\solution interface. In addition, once a regular CTAB monolayer was completely formed at the hydrophobic surface, the adsorption/desorption rate of CTAB was slow. This conclusion was supported by the fact that the reduction of iodine atoms on oxidized thyroxine occurred only at CTAB concentrations higher than 1.0 X 10(-5) M. (C) 2004 Elsevier B.V. All rights reserved.