A simple procedure to reveal the distinctive features of the polysaccharide polymer structure in bulk based on using heavy water (D2O) at small-angle neutron scattering (SANS) is proposed. The use of heavy water allowed us to increase the contrast of the internal structure of the samples capable of taking up this solvent and to trace structural changes depending on the degree of swelling in water vapour. As an example of this technique, SANS was used to study the structural characteristics of chitin-cellulose films. The results were suggestive for conclusions about the inner structure of these films and showed that the proposed approach in combination with SANS technique is an efficient and useful tool for structural characterisation of polymer materials in a solid state.