The present invention relates to a process for the preparation of a polysaccharide composed of D-xylose units of formula (III) linked together via beta 1,4 glycosidic bonds wherein R1 is hydrogen or acetyl, R2 is hydrogen, acetyl or a 4-O-methyl glucuronic acid unit, wherein, when R2 is a 4-O-methyl glucuronic acid unit, the R1 group on the same saccharide unit is defined as G, wherein G is hydrogen or acetyl, and wherein the sugar unit at the reducing end of sun such polysaccharide is xylose, lyxose or xylulose, said process comprising the following steps: selective deacetylation of xylan extracted from beech wood and isomerization of the selectively deacetylated xylan achieved in step or the following steps: isomerization of xylan extracted from beech wood and selective deacetylation of isomerized xylan achieved in step. The process is useful for the preparation of pentosan polysulfate or pharmaceutically acceptable salts thereof for pharmaceutical use.
