Knowledge of oxygen consumption related to oxygen distribution in biofilms is crucial to understand what is really happening in the biofilm endogenous metabolism. In this article, mature biofilms were taken from a municipal wastewater treatment plant for lab-scale experiments. A pH 7 buffer solution was used as influent instead of wastewater, indicating the absence of an external input of substrate. Oxygen distribution was measured in the next approximately 130 h at different locations in the biofilm. Two distinct trends in oxygen profiles in the biofilm were observed during endogenous respiration. In the first period under starved condition, oxygen penetrated more and more deeply in the biofilm with time, which is related to the smaller endogenous oxygen uptake rate. This phenomenon agrees with the traditional endogenous respiration theory developed in activated sludge systems. However, opposite trends were observed as the time passed by, where oxygen penetration became shallower again. A diffusion-reaction model concerning the generation and utilization of electron-donor substrate during endogenous respiration was proposed and agreed well with the changes in the measured oxygen profiles. (C) 2013 Elsevier B.V. All rights reserved.