A gas diffusion electrode system that can supply gaseous substrates to enzymes from the gas phase in bioelectrocatalysis is essential to increase the mass-transfer of gaseous substances; such a system can also address the problem of mass-transfer-limited reactions for gaseous substrates with low solubility in the dissolved system. We have constructed gas diffusion dioxygen (O-2)-reducing biocathodes with bilirubin oxidase (BOD) as an electrocatalyst. Novel carbon nanotubes (CNTs) with hollow structures were used as a carbon material to increase the gas permeability. We achieved a steady-state current density of more than 30 mA cm(-2) at pH 5 in direct electron transfer-(DET-) type bioelectrocatalytic O-2 reduction under quiescent conditions. The significance of the structural characteristics of the hollow CNTs was discussed based on microscopic observations. The hollow CNTs also provided platforms for mediated electron transfer-(MET-) type bioelectrocatalysis of BOD (not only in monolayer but in multilayers). MET-type reaction of BOD proceeded effectively at pH 7.0, at which the DET-type activity decreased down to one third of that at pH 5.0, and can cover a weak point of BOD as a decrease in the enzyme activity at neutral pH. (C) 2017 Elsevier Ltd. All rights reserved.