Flow injection systems with immobilized enzymes for the parallel determination of chemically similar substances such as xylose/glucose and L-leucine/L-isoleucine/L-valine are described. The determination of xylose and glucose was performed either by differential measurement or by an ’enzyme-array’. Because of the different linear ranges of the calibration curves for xylose and glucose quantification was only possible when there was an excess of xylose (xylose 0.1-2.5 mmol dm(-3), glucose 0.001-0.1 mmol dm(-3)). The parallel quantification of the branched-chain amino acids could be realized by an array arrangement using an FIA-system with several immobilized enzymes of slightly different selectivity. The total amount of branched-chain amino acids can be up to about 1.1 mmol dm(-3).