Abstract: A purpose-built flow injection analysis system was constructed that incorporated a 25 µL reactor containing xylose isomerase (omitted in determination of xylose alone), aldose 1-epimerase and glucose dehydrogenase immobilized on CPG-10 glass beads (37 to 74 µm diameter; 51.5 nm pore size). The NADH produced from NAD+ incorporated in the carrier solution [0.1 M phosphate buffer (pH 7.0) containing Mg] was detected electrochemically with use of a dye-modified graphite electrode in a wall-jet configuration (cf. Appleqvist et al., Anal. Abstr., 1985, 47, 12D151). Up to 30 determinations per hour were possible and calibration graphs were rectilinear for up to 2 mM xylose or -xylulose.