Spatioselectivity & mechanisms of reaction. Small-angle X-ray scattering (SAXS) experiments were carried out to detect the effects of the acetylation treatment on the morphology of nanoscale cellulose fibril structures. The results showed clear and systematic differences due to the treatment and its harshness. In the dry state, the elementary fibrils stayed further apart from each other in the acetylated samples compared to the reference paper. When the samples were saturated with water, the elementary fibril bundles in the reference sample swelled more than those in the acetylated samples. These systematic differences, due to the acetylation treatment observed both in wet and dry states, show that the acetylation treatment has a clear effect on the structure of the paper at the level of the elementary fibrils (diameter of some nanometres), indicating that the reaction took place at least partially at the elementary fibrillar surfaces, within the fibrillar bundles. Naturally the acetylation also occurs on the outer fibrillar bundle surfaces. To compare with the diffraction and NMR studies and illustrate the mechanism of acetylation, a fibrillar bundle geometry was prepared (Figure ) to contain different degrees of surface acetylation throughout the fibrillar bundle. Different mechanisms were assumed where: A) low DS regioselective 6-OH acetylation occurs at most fibril surfaces, except for some surfaces within fibrillar aggregates, and B) topochemical full fibril conversion to CTA occurs, as observed in the presence of pyridine, which is consistent with the acid-catalysed mechanism elaborated by Sassi & Chanzy . This latter mechanism at high DS (increasing pyridine contents), where full fibrils are converted to CTA, is also evident from the formation of a diffraction peak in the wide-angle region at a 2Q value of 9 o (Figure ), especially for the 25 vol% sample -this resembles the (010) plane from the CTA I literature structure ; Figure ) and is not associated with any native unhydrated crystalline cellulose allomorph. The longer interplanar distance corresponds to the extended separation of the repeating sugar units by the increased steric repulsion, between equatorial acetate groups -as opposed to the shorter distance unit separation by interchain equatorial H-bonds, in the native cellulose I allomorph. This diffraction intensity is at slightly shorter d-spacing than the literature structure and the Scherrer length (crystallite size) is clearly rather short (< 4 nm). This indicates deviations from the bulk crystal structure.