The water soluble polysaccharide isolated from rye flour gave on hydrolysis, Xylose (60%), arabinose (29%) and glucose (5%). After methylation of the hemicellulose, the sugars obtained on hydrolysis of the product were separated on a cellulose column, giving 2:3:5-tri-O-methyl-L-arabinose (5 parts), 2:3-di-0-methyl- D-xylose,(6) 2-0-methy1 -D-xylose (5) and a small amount of D-xylose. The general structure of the xylan is therefore similar to other known araboxylans, having a backbone of l:4 linked xylopyranose units with terminal arabinose units linked through position 3. In the isolation of the trisaccharide, Q-L-arabofuranosyl (1->3)- 0-D -xylopyranosyl (1->4) -D-xylopyranose, it has been shown that some of the arabinose units are in the form of single side chains. 4-0-Methyl-D-xylose has been synthesised from D-arabinose, and it has been shown to degrade in alkali giving mainly formic acid and xyloisosaccharinic acid, which is the expected product from sugars with 1:4-linked xylose residues. The action of alkali on various cereal xylans has been studied. The degradation products have been shown to include formic, lactic acids together xyloisosaccharinolactone and small quantities of xylometasaccharinolactones. Xylometasaceharinolactone has been found to be present in the hydrolysis products of the alkali stable rye polysaccharide. The mechanisms for the degradation and stopping reactions are discussed. Two unsuccessful attempts have been made to synthesise 1:5-dihydroxypentan-2:3-dione.