Cellulose-based nanocomposite films with different ratio of cellulose I and II were produced by means of partial dissolution of microcrystalline cellulose powder in lithium chloride/N,N-dimethylacetamide and subsequent film casting. The mechanical and structural properties of the films were characterised using tensile tests and X-ray diffraction. The films are isotropic, transparent to visible light, highly crystalline, and contain different amounts of undissolved cellulose I crystallites in a matrix of regenerated cellulose. The results show that, by varying the cellulose I and II ratio, the mechanical performance of the nanocomposites can be tuned. Depending on the composition, a tensile strength up to 240 MPa, an elastic modulus of 13.1 GPa, and a failure strain of 8.6% were observed. Moreover, the nanocomposites clearly surpass the mechanical properties of most comparable cellulosic materials, their greatest advantage being the fact that they are fully biobased and biodegradable, but also of relatively high strength. (c) 2005 Elsevier Ltd. All fights reserved.