AbstractFlow injection systems with immobilized enzymes for the parallel determination of chemically similar substances such as xylose/glucose andL‐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