Water and fuel crossover in a direct methanol fuel cell (DMFC) stack under various operating conditions were quantitatively determined by a method of mass balance analysis of fuel and water. With increasing the discharge voltage, the fuel crossover rate increases, but the water crossover decreases slightly. With increasing the operating temperature, both the water and fuel crossover rates increase significantly. The amount of water permeated across the electrolyte membrane is as much as eight times that of water produced by methanol electrochemical oxidation at the anode. There are three types of water crossover, including proton osmotic drag, methanol migration drag, and water spontaneous crossover. For a 24-cell DMFC stack that uses 1.0 M methanol and operates at 62 degrees C under 8 V discharge, the water crossover rates for proton osmotic drag, methanol migration drag, and water spontaneous crossover are 0.661, 0.243, and 1.186 mg min(-1) cell(-1) cm(-2), respectively. Among them, water spontaneous crossover is the most significant. There is a significant portion of water evaporation at the cathode outlet which is highly dependent on the operating conditions and relative humidity. Water crossover and evaporation result in great challenges for a DMFC system to recycle the cathode water. (C) 2008 The Electrochemical Society.