This keynote presented in the first International Conference on Applied Energy highlights the coupling phenomena of heat/mass and electrochemical reactions in fuel cells. With three selected examples from our recent studies of direct methanol fuel cells (DMFC), we show that a better understanding of heat and mass transport is important to improve the fuel cell performance and operating stability. The first example is about passive DMFCs (with neither liquid pumps nor gas compressors). We show that a change in the feed methanol concentration in a passive DMFC not only alters the rate of methanol transport, but also changes the cell operating temperature. As a result, the kinetics of both the methanol oxidation and oxygen reduction reactions change with methanol concentration. The second example is to show the coupling of oxygen transport and fuel cell operating stabilities. It is demonstrated that the DMFC equipped with the conventional serpentine fields usually suffers from serious discharging instabilities when the oxygen flow rate is low. We invented a so-called convection-enhanced serpentine flow field (CESFF) and found that with this new flow field the oxygen flow rate required to ensure a stable operation was significantly reduced. The application of CESFF enables a reduction in the parasitic power in the fuel-cell system and leads to a higher system efficiency. The third example is about the coupling of oxygen transport and electrochemical reactions in DMFCs. In conventional wisdom, the DMFC is an electrochemical device to generate electricity. However, we discovered that the DMFC can also be used to produce hydrogen besides its function of generating electricity. Copyright (C) 2010 John Wiley & Sons, Ltd.