The Aux of a product generated by an electrochemical reaction at the disc of a rotating ring-disc electrode is detected by the current of the product at the ring electrode after some transfer time. This transfer time is eliminated by deconvolution allowing to determine the flux simultaneously with its generation. The transfer function in the frequency domain required for deconvolution is measured experimentally. The validity of the procedure is tested. As an example, passive iron in berate buffer is chosen. During an anodic current pulse, the partial current generating iron(III) ions decreases with time and the partial current of film growth increases until a steady state is established after several seconds.