Abstract A new acidic sidegroup in xylans, from both kraft pulp and pulping liquor, was identified by NMR spectroscopy. Unmodified oligosaccharides from kraft pulp xylan were obtained by enzymatic hydrolysis with xylanase (Trichoderma reesei). The acidic oligosaccharides were separated from the neutral forms on an anion exchange resin. The new acidic sidegroup was identified as 4- deoxy-β- l-threo- hex-4- enopyranosyluronic acid (hexenuronic acid) by 1H and 13C NMR spectroscopy. Hexenuronic acid is a β-elimination product of 4- O-methylglucuronic acid and is formed during kraft pulping. HMBC and NOESY experiments showed that hexenuronic acid is attached β-(1 → 2) to xylose. The NOESY data further indicated that hexenuronic acid protrudes from the main xylan chain. The p K a values for hexenuronic acid (3.03) and 4- O-methylglucuronic acid (3.14) attached (1 → 2) to xylose were determined from pH-dependent chemical shifts.