Redox protein cytochrome c was immobilized at high electrochemically accessible loading on mesoporous films of antimony doped tin oxide (ATO) exhibiting high conductivity, transparency and a large surface area. The grafting was achieved by covalent attachment of the protein to the electrode surface. Alternatively, cytochrome c was effectively adsorbed on the ATO electrode surface due to strong electrostatic interaction between the positively charged cytochrome c and the negatively charged ATO surface. The amount of electrochemically addressable cytochrome c is proportional to the specific surface area, reaching up to 440 pmol/cm(2) and 600 pmol/cm(2) for covalently attached and adsorbed protein, respectively, for the 370 nm thick films. The covalently attached protein exhibits substantially higher stability towards leaching than the adsorbed one. The combination of transparent conducting porous electrode matrix with the electroactive proteins is promising for the development of efficient bio-optoelectronic devices and for in situ spectroelectrochemical studies of biomolecules. (C) 2013 Elsevier Ltd. All rights reserved.