The surface of a gold (Au) electrode was modified with 4-mercaptophenylboronic acid (MPBA) and dithiobis(4-butyrylamino-m-phenylboronic acid) (DTBA-PBA) to prepare sugar-sensitive electrodes. MPBA and DTBA-PBA formed well-packed monomolecular layers on the Au electrode through a sulfur-Au bond. The MPBA- and DTBA-PBA-modified electrodes exhibited a nearly reversible cyclic voltammogram (CV) for the Fe(CN)(6)(3-/4-) ion in acidic solution, while the CVs were significantly attenuated in alkaline media as a result of addition of OH- ion to the boron atom to generate the negatively charged surface. In other words, the negatively charged monolayers blocked the surface of the Au electrode from the access of the Fe(CN)(6)(3-/4-) ion. The pK(a) values of the addition equilibrium of the OH- ion to the MPBA and DTBA-PBA monolayers were estimated to be 9.2 +/- 0.1 and 8.0 +/- 0.2, respectively, on the basis of the pH-dependent peak current (i(p)) in the CV of the Fe(CN)(6)(3-/4-) ion. On the other hand, in the presence of sugars, the addition of the OH-ion was accelerated by forming the phenylboronate esters of sugars on the surface of the monolayers. The pK(a) values for the MPBA and DTBA-PBA monolayers were 8.3 +/- 0.1 and 7.2 +/- 0.1, respectively, in the presence of 50 mM D-fructose. The MPBA- and DTBA-PBA-modified electrodes can be used for determining sugars on the basis of the change in i(p) of the Fe(CN)(6)(3-/4-) ion in the presence of sugars. The calibration curves useful for determining 1-100 MM D-glucose, D-mannose, and D-fructose were obtained.