The operational characteristics of a small-scale passive air-breathing direct methanol fuel cell (PAB-DMFC) are comprehensively investigated under both steady-state and dynamic conditions. As the most important operating parameter, methanol concentration has significant effects on the cell performance. For different methanol concentrations (e.g., 0.5 and 8 M), the structural adaptations are particularly discussed. The results show that the structural factors are closely related to the influence degree of methanol concentration. In this study, the characteristics of the open circuit voltage (OCV) under various structural and methanol-concentration conditions are presented. Besides, the effects of other operating conditions such as running time, forced air convection and refueling action on the cell performance are also evaluated. In addition, a series of dynamic operations of the PAB-DMFC are conducted under different load cycles. Accordingly, the transient phenomena such as voltage undershoot and overshoot are explored. A fundamental principle for evaluating the operational characteristics of a PAB-DMFC is to simultaneously take into account the mass transfer requirements such as reactant delivery, product removal, methanol/water crossover control and so on. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.