In the present work, we report the design, fabrication and evaluation of a membraneless mixed-reactant and air breathing microfluidic direct methanol fuel cell (ML-mu DMFC) stack operated in passive mode. The operation under mixed-reactant conditions was achieved by using a highly methanol-tolerant Ag/Pt/CP cathode with ultra low Pt loading in alkaline medium. Prior to the fabrication of the stack, a flow simulation was made in order to study the behavior of the reactants stream in the microchannel through the 2 cells. Subsequently, the device was tested in passive mode using a mixture of 5 M MeOH + 0.5 M KOH. The results showed that by connecting the 2 cells in series, it is possible to effectively double the voltage of a single ML-mu DMFC, as well as increasing the absolute power by 75% with practically no cost increase. The stack was capable of operate continuously for more than 2 h with a single charge of 40 mu L, producing an OCV of 0.89 V and a maximum power density of 3.33 mW mg(pt)(-1). Additionally, the device exhibited good stability throughout a 10 h test.