Abstract: A method for the independent, reversible, optical determination of the pH value and the ionic strength of an aqueous sample with the aid of two different sensors in accordance with the fluorescence method, in which method two optical sensors, which are each composed of polymers of different structure but each contain the same fluorescent dye and which each consist of a coated material composed ofa) a carrier material, to which there is appliedb) at least one water-insoluble layer of a polymer comprising at least one hydrophilic monomer (A) from the group of substituted olefins, andc) a proton-sensitive fluorescent dye which is bonded directly or via a bridge group to the spine of polymer b) or which is incorporated in polymer b),are brought into contact with an aqueous test sample, irradiated with exciting light, the fluorescence is measured and the pH values and the ionic strengths are calculated from the measured fluorescence intensities with reference to calibration curves.