Air maldistribution is frequently encountered in direct methanol fuel cell (DMFC) stacks. To understand the characteristics of different single cells at air maldistribution, the effect of the oxygen stoichiometry (OS) on the behavior of a single DMFC in galvanostatic operation is investigated both experimentally and numerically. Special attention is paid to the cell potential dependence on the OS, based on which the OS is characterized into three distinct ranges. The cell potential varies little in range 1, but decreases dramatically with the OS in range 2, and becomes negative in range 3. The cell characteristics in each range are highlighted in detail. The entire cell acts as a normal DMFC in range 1, but turns to bi-functional mode in range 2 with parasitic hydrogen evolution from the negative electrode (NE) due to partial oxygen depletion. Extremely high current density arises near the air inlet when the cell potential approaches zero, which can lead to high electrode potential in the NE and induce serious ruthenium dissolution. When the OS decreases further to range 3, hydrogen evolution starts to occur in the positive electrode (PE), and the evolved hydrogen can be re-oxidized upstream, leading to coexistence of four electrochemical half-reactions in the PE. (C) 2013 Elsevier Ltd. All rights reserved.