Regenerated cellulosic fibres and comparative materials were studied in the hydrated state by low-resolution proton NMR. Experiments at variable pH and temperatures have shown that the shortened T-2 relaxation times of water within fully swollen cellulosic fibres are dominated by proton exchange with accessible cellulose hydroxyl groups. Proton exchange is accelerated by both acid and base catalysis, with relaxation data used to estimate rate constants for acid, base and neutral mechanisms. Complementary deuterium exchange measurements suggest that accessible cellulose regions below the immediate water interface may not contribute effectively to the proton exchange relaxation mechanism, with two-site relaxation models sensitive only to the direct pore surface area. Differences between surface-relaxing water and deuterium-exchanging water can therefore be used to determine an apparent depth of the accessible cellulose, which is greater for viscose and modal compared to lyocell. However, from relaxation data lyocell has a higher pore surface area. This work also confirmed that water interacting with accessible cellulose experiences motional restriction, allowing an intra-molecular dipolar contribution to relaxation. However, in the fully swollen state water molecules are diffusing rapidly between all internal fibre environments and there is no evidence of specific binding. (C) 2008 Elsevier Ltd. All rights reserved.