Аннотация:The changes in the oxidizing properties of redox mediators induced by light flashes and resulting from their interaction with the photosynthetic electron transport chain carriers were studied potentiometrically by following the oscillations in the redox potential value in a suspension containing a photosynthesizing object and a redox mediator. The conditions for potentiometric measurements were determined. The method described is highly sensitive, allows following redox conversions of the mediators which do not change their color during redox conversion and is instrumentally simple. [Rhodopseudomonas sphaeroides was used.].