Enzyme substrates in aqueous liquids are continuously measured by introducing a sample containing the substrate to be determined into a current of buffer solution, reacting the substrate on an immobilized enzyme and measuring, in a measurement chamber, a physical change in the solution brought about by the reaction, wherein a current of buffer solution containing the sample to be determined is passed alternatingly (i) directly through the measurement chamber with the production of a reference signal and (ii) first over the immobilized enzyme and thereafter through the measurement chamber with the production of a measurement signal, and relating the reference signal to the measurement signal to indicate the enzyme substrate initially present.