While the impact of acetylation on increased durability is clear, the mechanisms at the molecular level that contribute to this durability is an active area of research with competing hypotheses. Hydration is clearly involved somehow, as increasing acetylation as measured by weight-percent gain (WPG) reduces the moisture content within plant cell walls (Fig. ), which had been previously hypothesized by prior X-ray fluorescence microscopy (XFM) measurements. Comparing ion uptake and diffusion between acetylated and non-acetylated wood samples determined by XFM, the acetylated wood samples are slower but Fickian, as we would expect for simple diffusion. One proposed hypothesis to explain these trends is that the acetyl groups make specific interactions with ions that would retard diffusion, keeping the ions away from solution. Another potential rationale for slower diffusion is the reduced hydration induced by increased acetylation, as lower moisture content within secondary cell walls has been associated with reduced diffusion and structural transitions by both experiment and simulation. The molecular level picture of a plant cell wall provided by molecular dynamics (MD) simulations (Fig. ) is well positioned to discriminate between these two alternatives, with many examples in the literature where MD provides a molecular picture of the nanoscale interactions that control cell wall structure. 9,37-40 By simulating both a set of simulation conditions when acetylation takes place but the number of water molecules remains fixed, creating an expanded cell wall and a independent simulation set where acetylation directly replaces water molecules to create a constant size cell wall, we can determine if acetyl-ion interactions are the primary source of altered diffusion, or if instead reduced hydration is the primary culprit. Whereas diffusion is anticipated to reduce substantially upon acetylation, we find that this only occurs when acetylation displaces water molecules filling in space between the cell wall polymers. We measure these water pockets directly from simulation, as well as other quantities of interest to fully quantify the structural and dynamical changes within secondary cell walls upon acetylation.