Particles of chitosan have a very polar surface, and thus they are easily wetted by water and also dissolve in acidic media. The hydroxyl and amino groups offer a high potential for grafting reactions. Aldehydes and carbonic acid derivatives were covalently grafted, preferably onto the amino groups. The reactions were carried out in solution as homogeneous phase reactions as well as on the particle surfaces as heterogeneous phase reactions. Covalently bonded alkyl chains impart the chitosan molecules with hydrophobic properties. We employed different methods such as NMR, XPS, elemental analysis and FT-IR spectroscopy to study the reactions and to estimate the degree of functionalization. The wetting behavior of the particles was investigated by a modified Wilhelmy technique, where an adhesive tape completely coated with the particles was dipped in water. Some of the samples having a high degree of functionalization showed super-hydrophobic surface properties. The observed super-hydrophobic effect results from a combination of the hydrophobic properties of the modified particles and the roughness of the particle coating. (C) Koninklijke Brill NV, Leiden, 2009