The potentiometric analysis of peroxyacetic acid (PAA) in the presence of a large excess of hydrogen peroxide (H2O2), up to 500 times the concentrations of PAA, has been presented for the first time. This method is based on the detection of the potential change of the glassy carbon indicator electrode in the I-/I-2 potential buffer which is caused by the change in the concentrations of I- and I-2 as a result of the redox reaction of the PAA and/or H2O2 with I-. Based on the fact that the reaction rate of PAA and I- is much faster than that of H2O2 and I-, a high selective response for PAA was obtained. The different factors, such as the concentrations of I- and I-2 in the potential buffer, affecting the selectivity and sensitivity have been studied. A good calibration curve for PAA, the slope of which is in close agreement with that expected from the Nernst equation, i.e., 29.6 mV per decade, was obtained with a correlation coefficient higher than 0.993. The detection limit for PAA was found to be in the micromolar range depending on the concentration of the coexistent H2O2. (C) 2003 Elsevier Science B.V. All rights reserved.