The interaction between xylan and cellulose microfibrils is important for secondary cell wall properties in vascular plants. However, the molecular arrangement of xylan in the cell wall and the nature of the molecular bonding between the polysaccharides are unknown. In dicots, the xylan backbone of β-(1,4)-linked xylosyl residues is decorated by occasional glucuronic acid and approximately one half of the xylosyl residues are O-acetylated at C-2 or C-3. We recently proposed that the even periodic spacing of GlcA residues in the major domain of dicot xylan might allow the xylan backbone to fold as a 2-fold helical screw to facilitate alignment along, and stable interaction with, cellulose fibrils (Bromley et al. 2013). However, such an interaction might be adversely impacted by random acetylation of the xylan backbone. Here, we investigated the arrangement of acetyl residues in Arabidopsis xylan using mass spectrometry and NMR. Alternate xylosyl residues along the backbone are acetylated. Using molecular dynamics simulation, we found that a 2-fold helical screw conformation of xylan is stable in interactions with both hydrophilic and hydrophobic cellulose faces. Tight docking of xylan on the hydrophilic faces is feasible only for xylan decorated on alternate residues and folded as a 2-fold helical screw. The findings suggest an explanation for the importance of acetylation for xylan–cellulose interactions, and also have implications for our understanding of cell wall molecular architecture and properties, and biological degradation by pathogens and fungi. They will also impact strategies to improve lignocellulose processing for biorefining and bioenergy.This article is protected by copyright. All rights reserved.