Porous composite films containing cellulose nanofibrils (from sisal) and TiO2 nanoparticles (ca. 6 nm diameter) are obtained in a layer-by-layer assembly process. Each layer consists of ca. 0.18 mu g cellulose nanofibrils and ca. 0.72 mu g TiO2 (determined by QCMB) and adds a thickness of ca. 16 nm (by AFM) to the uniform deposit. The TiO2 nanophase is creating conducting pathways for electrons in a relatively open cellulose structure (ca. 82% open pores) potentially suitable for the immobilization of large redox proteins such as methemoglobin. Methemoglobin is shown to readily adsorb into the cellulose-TiO2 film. However, electrochemical responses for the immobilized methemoglobin in aqueous 0.1 M phosphate buffer at pH 5.5 suggest that facile demetallation occurs. Experiments with Fe3+ in the absence of protein result in voltammetric responses indistinguishable from those observed for immobilized methemoglobin. In the presence of ethylenediamine tetraacetic acid (EDTA) the voltammetric signals for the Fe3+ immediately disappear. Complementary experiments conducted in 0.1 M acetate buffer at pH 5.5 demonstrate that methemoglobin can indeed be immobilized in electrochemically active form and without demetallation loss of the voltammettic signal in the presence of EDTA. Demetallation appears to occur (i) in the presence of phosphate, (ii) at pH 5.5 (iii) and in the presence of a charged surface. (c) 2007 Elsevier B.V. All rights reserved.