The structure of one of the hemicellulose A fractions (HA-1) extracted from the seeds of the grape variety Palomino (Vitis vinifera cv. Palomino) has been studied by means of methylation, 1H NMR, 13C NMR, and partial hydrolysis. This hemicellulose seems to be a homogeneous polysaccharide with an apparent molecular weight of 68 500. Its structure is that of an acidic xylan, a linear chain of beta-D-xylopyranosyl units, bonded together by (1-->4) glycosidic links, containing single alpha-D-xylopyranosyl and 4-O-methyl-alpha-D-glucopyranuronosyl residues joined by glycosidic links to position 2 of the xylose units of the main chain, in proportions of one residue to every 25 units of xylose.