Arabidopsis xylan consists of a linear chain of β-1,4-linked D-xylosyl residues, about 10% of which are substituted with single residues of α-D-glucuronic acid (GlcA) or 4-O-methyl-α-D-glucuronic acid (MeGlcA) at O-2. In addition, about 60% of xylosyl residues are acetylated at O-2 and/or O-3. Previous studies have identified a number of genes responsible for elongation of the xylan backbone, addition of the GlcA substituents, and methylation of the GlcA residues. Yuan et al. (2013) have recently reported that the 2-O- and 3-O-monoacetylation of xylosyl residues in Arabidopsis xylan requires a DUF231 domain-containing protein, ESKIMO1 (ESK1), and proposed that ESK1 and its homologs are putative acetyltransferases responsible for xylan acetylation. It was noticed that the (1)H nuclear magnetic resonance (NMR) spectra of the acetylated xylan from the esk1 mutant and the wild-type Arabidopsis exhibited a prominent proton signal peak at 5.42 ppm in addition to resonances corresponding to known acetylated structural groups of xylan. Here, we performed detailed structural investigation of wild-type Arabidopsis acetylated xylan using 2-dimensional (1)H- (1)H and (1)H- (13)C NMR spectroscopy and found that the signal peak at 5.42 ppm in the (1)H NMR spectrum was attributed to GlcA residues substituted at O-2 with α-D-galactose (Gal), indicating the presence of Gal-GlcA disaccharide side chains in Arabidopsis xylan. This finding was further supported by analysis of endoxylanase-digested xylan using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Our study demonstrates that Arabidopsis xylan contains Gal-GlcA disaccharide side chains in addition to GlcA, MeGlcA, and acetyl substitutions.