Direct electron transfer (DET)-type bioelectrocatalysis is an essential technique for constructing simple bioelectrochemical devices such as biosensors, bioreactors, and biofuel cells. Bilirubin oxidase (BOD), a biocatalyst for the four-electron reduction of dioxygen (O-2) into water, is a promising enzyme for powerful DET-type biocathodes. Mesoporous carbon materials are often used in BOD-modified biocathodes. However, the supply of O-2 becomes a key, factor governing BOD-catalyzed current densities owing to its low solubility. In this study, we analyzed steady-state rotating disk voltammograms of a BOD-catalyzed reaction in a rigid manner by taking into account the mass-transport as well as the enzymatic and the interfacial electron transfer kinetics. Non catalytic redox signals of adsorbed BOD were also analyzed to obtain the surface redox properties of BOD. The analysis revealed that modification of electrodes with bilirubin and/or negatively charged carbon nanotubes to improve the DET-type catalytic performance increased the amount of BOD molecules with the proper orientation for bioelectrocatalysis. The interfacial electron transfer kinetic characteristics remained almost unchanged. (C) 2016 Elsevier B.V. All rights reserved.