Method for determination of product and substrate concentrations in a medium

A method for determining substrate and product concentration in liquid and/or gaseous media in which several samples of at least one substance to be analyzed-the analyte-are removed in at least one sampling region (3) by time-controlled diffusion of the at least one analyte between the respective medium and a diffusion medium which is fed to the sampling regions (3) through fluid conduits (5a, 5b) using at least one pump (6) by semipermeable membranes (2) and subsequently the diffusion medium is transported to at least one detector (7) while simultaneously new diffusion medium is being fed from the sampling region (3) and is analyzed by this to determine the analyte concentration, characterized in that the detector provides a temporal concentration distribution or a temporal distribution of a signal proportional to the concentration, in that a change in the ratio of the signal maximum to the base line is ascertained in the output of the detector signal, and on the basis of this, a change in the diffusion properties of the semipermeable membrane is inferred and a correction factor is ascertained.