Investigations on the mass and neutron scattering densities of acidic polysaccharides are reported. The neutron scattering length densities of neutral and acidic sugars are sensitive to chemical composition, and are almost always higher than those of proteins. These findings were based on crystal data, and were confirmed by preliminary neutron scattering experiments, which gave matchpoints of 52% 2H2O for hyaluronate and 60 ± 5% for chondroitin sulphate. Densitometric studies gave the partial specific volumes v̄ of 0.58 ml g-1 for hyaluronate and 0.53 ml g-1 for chondroitin sulphate, and these could be completely interpreted in terms of partial specific volumes from crystal data and involving electrostriction. Neutron scattering studies on the hyaluronate binding region of proteoglycan show that it has a scattering density equivalent to 45% 2H2O. Densitometry gave a partial specific volume (v̄) of 0.66 ml g-1. Both can be accounted for by the above studies of v̄ for sugars and published values of v̄ for amino acid residues. A molecular weight of 81,000 ± 20,000 is found from neutron scattering, in satisfactory agreement with biochemical estimates of 83,000. The amino acid and sugar composition of the binding region is reported. The radius of gyration (RG) of the binding region at infinite contrast is 42 ± 2 Å. showing that the binding region in 0.2 m-NaCl is elongated and globular. The dependence of RG on the solvent contrast shows that the carbohydrate content is on the surface of the molecule with a higher scattering density than that of the protein. The Stokes radius of 66 Å from quasielastic light-scattering shows that the binding region is strongly hydrated, and is consistent with an oligosaccharide structure that is extended semi-rigidly into the solvent and traps much water. The globular nature of the protein component is further demonstrated by the observation of slowly exchanging NH backbone protons in its nuclear magnetic resonance spectrum, and most of these are exchanged on heating to 50 °C. There was no change in the RG of the binding region in H2O when a ligand of hyaluronate(dodecasaccharide, HA12) was added to it. This suggested that the shape of the binding site for HA12 was preformed. © 1981.