The distribution of 4-O-methylglucuronic acid residues along the polysaccharide chains of xylans isolated from birch, aspen, spruce, pine, and larch was studied here employing matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis of the oligosaccharide mixtures obtained by partial acid hydrolysis. The hydrolyzates thus obtained were analyzed by MALDI-MS or by capillary electrophoresis as well as by size exclusion chromatography in combination with MALDI-MS. In the case of all softwood xylans examined, the major portion of the 4-O-methylglucuronic acid residues were distributed regularly on every seventh or eighth xylose residue, while a minor portion of these residues were attached to adjacent xylose residues in the beta-(1-->4)-D-xylopolysaccharide chains. In contrast, the 4-O-methylglucuronic acid residues in the hardwood xylans examined were found to be distributed irregularly within the xylan. Further support for these experimental findings with the xylans was obtained by simulation of the distribution patterns of the molecular masses of oligosaccharides arising from polysaccharides with the postulated distribution of uronic acid residues.