A xylan containing 11.8% uronic acid and 16.9% O-acetyl has been isolated from birch (Betula verrucosa and B. pubescens) holocellulose by extraction with dimethyl sulphoxide. Methylation studies of the acetyl xylan itself and of a deacetylated sample have shown that the O-acetyl groups are attached to the xylose residues mainly in the 3-position and only to a lesser extent in the 2- position.