Abstract: A process for the quantitative determination of oxidizing and reducing substances is described, also mixed with chemically similarly reacting substances, particularly peracetic acid mixed with further substances such as hydrogen peroxide (H.sub.2 O.sub.2) and/or acetic acid by means of per se known potentiostatic amperometry, in which the determination takes place with a test electrode voltage, which, independently of the prevailing reaction mechanism, is below the diffusion limiting current range between the no-load voltage and the flex point of the current density-voltage curve. The peracetic acid determination takes place at a current density of <5%, preferably <1% of the limiting current density.