Cytochromes b 561 constitute a novel class of proteins in eukaryotic cells with a number of highly relevant common features including six transmembrane α-helices and two haem groups. Of particular interest is the presence of a large number of plant homologues having putative ascorbate- and monodehydroascorbate radical-binding sites. We conducted a diethylpyrocarbonate-modification study employing Zea mays cytochrome b 561 heterologously expressed in Pichia pastoris cells. Pre-treatment of cytochrome b 561 with diethylpyrocarbonate in oxidized form caused N-carbethoxylation of His86, His159 and Lys83, leading to a drastic inhibition of the electron transfer from ascorbate. The activity was protected by the inclusion of ascorbate during the treatment. However, midpoint potentials of two haem centres did show only slight decreases upon the treatment, suggesting that changes in the midpoint potentials were not the major cause of the inhibition. Present results indicated that Zea mays cytochrome b 561 conducted an ascorbate-specific transmembrane electron transfer by utilizing a concerted H+/e− transfer mechanism and that the specific N-carbethoxylation of haem axial His86 that would inhibit the removal of a proton from the bound ascorbate was a major cause of the inhibition. On the other hand, Lys83 might be important for an initial step(s) of the fast electron acceptance from ascorbate.