The possible role of structural iron in clays to promote direct electron transfer of hemoglobin (Hb) was investigated. Clays containing different amounts of iron situated in octahedral or tetrahedral sites have been used to modify glassy carbon electrodes: nontronite, synthetic montmorillonite, and saponite. A synthetic montmorillonite containing noniron impurities was used as a reference. Interactions between Hb and these days were studied with the establishment of adsorption isotherms and by the analysis of X-ray diffraction patterns, FTIR, and UV-vis spectra of the Hb-day samples. The electrochemical behavior of clay modified electrodes (CME) was characterized by cyclic voltammetry in the presence of Hb in solution or adsorbed within the days. Nontronite, which contains the highest amount of structural iron, enhanced significantly direct electron transfer of Hb. Finally, the electrocatalytic behavior of Hb-Nontronite CME in the presence of hydrogen peroxide was also studied, and the H2O2 calibration curve was recorded under amperometric conditions for different bioelectrode configurations.