The objective of this study was to investigate the application of microelectrodes with electrochemical impedance spectroscopy (EIS) for distinguishing between one- and two-electron transfer electrochemical reactions occurring within marine microbial biofilms. In our study, both gold and gold-mercury arnalpin microelectrodes with tip diameters of 100 mu m were used. EIS measurements were performed at the halfwave potentials of the chemicals of interest in 0.5 M NaCL aqueous solutions containing 100 mu M each of FeCl3, [Fe(Cat)(2)](-), [Fe(Cat)(3)](3-), [Fe(4NCat)(3)](3-), MnCl2, FeCl2 and mixed solutions with two of those chemicals in NaCl solution or seawater. The results showed that there was a linear relationship between the impedance Z(re), Z(im) and anglular frequency omega(-1/2) obtained at phase angle of 45 degrees for both one- and two-electron transfer reactions. One-electron transfer reactions could then be distinguished from two-electron transfer reactions by the slopes. (c) 2005 Elsevier Ltd. All rights reserved.