Methemoglobin (bovine) is immobilized from aqueous phosphate buffer (pH 5.5) solution into thin porous TiO2 (anatase) films at ITO electrode surfaces. Films of TiO2 are produced in a deposition process employing 40 nm diameter TiO2 nanoparticles suspended in dry methanol followed by calcination. The pore size in these films is sufficient for methemoglobin (ca. 6 nm diameter) to diffuse into the porous structure (over several hours) and to remain immobilized in electrochemically active form. The electrochemical reduction of methemoglobin immobilized in TiO2 and immersed in aqueous phosphate buffer at pH 5.5 is observed in two steps with (i) a small quasi-reversible voltammetric response at -0.16 V vs. SCE (Process 1) and (ii) an irreversible reduction peak at ca. - 0.5 V vs. SCE (Process 2). The irreversible response is recovered only after slow chemical re-oxidation of hemoglobin to methemoglobin. At sufficiently negative applied potential "electrochemical doping" of the TiO2 host is observed to lead to a considerably enhanced reduction Process 1. TiO2 can be temporarily switched from a non-conducting (irreversible electron transfer) into a conducting (reversible electron transfer) state. (C) 2006 Elsevier B.V. All rights reserved.