Xylanase A from alkaliphilic Bacillus halodurans C-125 was expressed in Escherichia coli and purified by affinity and anion exchange chromatographies. It exhibited a strong substrate inhibition using xylan as the substrate. Its K i value increased with an increase in pH. The effect of pH on the enzyme activity was determined using two aryl-xylobiosides as substrates, and it was found that the enzyme had a flat k cat-pH curve in the pH range of 5.8–8.8. This range was different from that obtained with 0.45% xylan as previously reported (Honda, H. et al., Agric. Biol. Chem., 49, 3165–3169, 1985). The substrate inhibition was presumed to cause the difference. It has been clarified that the use of aryl-xylobiosides as substrates yields more accurate kinetic results than that of xylan.