Variations of methanol concentration and solution volume in an anode water tank with operating time in a 20 W direct methanol fuel cell (DMFC) system that uses pure methanol and recycles the cathode water were simulated. The basic parameters for simulation of the DMFC system were acquired from a model DMFC stack. Many factors in a DMFC system design, such as the average cell voltage, operating temperature, water recycling percentage, and initial solution volume, affect water consumption, generation, and evaporation significantly, and in turn, cause gradual changes of methanol concentration and solution volume. The water generation rate varies significantly with operating temperature, but only slightly with average cell voltage. At 62 and 42 degrees C, the generation rates of recyclable water are 1.44 and 0.88 g/min, respectively, for a DMFC system using 1.0 M methanol in an anode water tank and operating at 8.0 V (or average cell voltage 0.33 V). To balance water consumption and generation, it is necessary to recycle 98% water at an average cell voltage of 0.42 V, and to recycle 90% water at an average cell voltage of 0.33 V for the DMFC system operating at 62 degrees C. Only 80% water needs to be recycled for the same DMFC system at an operating temperature of 42 degrees C and average cell voltage of 0.33 V. (C) 2008 The Electrochemical Society.