In accordance with the theory of contact exchange, it is hypothesized that the presence of negative charge in microbial exopolysaccharides increases the rate of cation transport. These typically acidic materials may provide a fast-track for the diffusion of nutrient cations through the polymer layer for uptake at the organism cell surface. We have measured the diffusion coefficient of a model cation, Mn2+ through xanthan, de-acetylated xanthan, scleroglucan and chitosan using spatially resolved NMR relaxometry. The concentration of Mn2+ in solution was measured by recording the change in the spin-spin (T-2) relaxation time of water H-1 over time in compartments either side of a polymer layer. This approach provides a sensitive, in situ, non-invasive method of measuring the rate of diffusion of paramagnetic cations through hydrophilic polysaccharides. The negatively-charged polysaccharides, xanthan and de-acetylated xanthan, permitted a significantly faster rate (2-2.5X) of cation transport compared to the uncharged polymer, scleroglucan. The positively-charged polysaccharide chitosan reduced the rate of Mn2+ diffusion to around half the value obtained for scleroglucan. These results suggest that the presence and nature of fixed charges on the polysaccharide molecule affects the rate of cation transport in accordance with the theory of contact exchange. The presence of negative charge on microbial exopolysaccharides may thus improve the availability of nutrient cations at the organism cell surface. (C) 2001 Elsevier Science Inc. All rights reserved.