The objective of this study is the development of bionanocomposites using cellulose microfibrils (CMF) as nanoreinforcement in two different biopolymers: poly(ethylene oxide) (PEO) and poly(vinyl butyral) (PVB). Nanocomposites were prepared using two different dispersion methods: dispersion in solvent and melt-extrusion. In order to study the state of dispersion, rheological, morphological and mechanical characterization were carried out. Rheological and morphological analysis showed that high dispersion of CMF was obtained though small differences were observed. Rheological results showed the formation of a CMF network at the rheological percolation (1.5-2 wt% CMF). Mechanical properties were also influenced by the presence of CMF, increasing with their content in the bionanocomposites.