Abstract: Method and apparatus for spectrophotometric investigation of products of electrochemical reactions in which a solution containing at least one dissolved initial product is directed essentially axially towards a rotating disc electrode surrounded by an optically translucent ring, the flow being reversed at the rotating electrode into an outwardly directed radial flow, and the concentration of the products formed at the electrode being determined by passing monochromatic light through the radial product flow and the optically translucent ring and by measuring the light absorption, wherein the measurement is performed under quasi-stationary conditions in that the current for the electrochemical reaction is permitted to flow without interruption for at least approximately 0.5 seconds before the measurement is performed.