Rodlike fragments of the polysaccharide cinerean were examined in H2O and D2O with small-angle X-ray and neutron scattering (SAXS and SANS). From measurements or the macroscopic scattering cross section on an absolute scale the mass per length of the rods has been determined. The SANS measurements in D2O yield a value very different from those obtained by other measurements (SAXS with H2O and D2O; SANS with H2O). This effect has been used to determine the exchange of protons and bound H2O from the polysaccharide against deuterons and D2O. The number of H2O molecules per repeating unit is determined to be about 17.