A 1D + 1D model of a direct methanol fuel cell (DMFC) is developed. Analytical solution for the case of equal oxygen and methanol stoichiometries l is obtained. The model predicts a new effect: formation of a narrow zone of local current (bridge), which "short-circuits" cell electrodes at vanishingly small current in the load. The bridge arises only under nonzero flux of methanol through the membrane. This effect explains dramatic decrease of DMFC open-circuit voltage known as mixed potential. The expression for voltage loss at open-circuit E-0 is derived; E-0 increases with the rate of crossover and tends to zero with the growth of l. Numerical estimate for typical cell gives E-0 ≃ 500- 700 mV, which agrees with the experimental data. The model suggests a method for measuring mean crossover flux through the membrane and indicates conditions for cell operation with zero crossover, regardless of the transport properties of the membrane © 2005 The Electrochemical Society. [DOI: 10.1149/1.1905963] All rights reserved.