alpha-Glucuronidase is a very important enzyme for the complete hydrolysis of plant hemicellulose, but the substrate specificity of the enzyme has not previously been reported. In this connection, the three regioisomers of O-(alpha-D-glucopyranosyluronic acid)-D-xylose (aldobiouronic acid), 2-O-(alpha-D-glucopyranosyluronic acid)-D-xylose (13), 3-O-(alpha-D-glucopyranosyluronic acid)-D-xylose (14), and 4-O-(alpha-D-glucopyranosyluronic acid)-D-xylose (15), were stereoselectively synthesized to clarify the substrate specificity.