In this paper, design criteria and development techniques for planar air breathing direct methanol fuel cell stacks are described in detail. The fuel cell design in this study incorporates a window-frame structure that provides a large open area for more efficient mass transfer and is modular, making it possible to fabricate components separately. The membrane electrode assembly and gas diffusion layers are laminated together to reduce contact resistance, which eliminates the need for heavy hardware. The composite current collector is low cost, has high electrical conductivity and corrosion resistance. In the interest of quick and cost-efficient prototyping, the fabrication techniques were first developed on a single cell with an active area of 1.0 cm(2). Larger single cells with active areas of 4.5 and 9.0 cm(2) were fabricated using techniques based on those developed for the smaller single cell. Two four-cell stacks, one with a total active area of 18.0 cm(2) and the other with 36.0 cm(2), were fabricated by inter-connecting four identical cells in series. These four-cell stacks are suitable for portable passive power source applications. The performance analysis of single cells as well as stacks is presented. Peak power outputs of 519.0 and 870.0 mW were achieved in the stacks with active areas of 18.0 and 36.0 cm(2), respectively. The effects of methanol concentration and fuel cell self-heating on the fuel cell performance are emphasized. (c) 2006 Elsevier B.V. All rights reserved.