A passive fuel delivery system utilizing a natural-circulation mechanism for direct methanol fuel cells (DMFC) is presented in this paper. It was shown that this novel methanol solution feed system was capable of achieving a performance comparable to that of an actively pumped system for different methanol concentrations ranging from 0.5 to 4.0 M. The theoretical and experimental results showed that the natural-circulation feed flow rate increased with the increase in current density. As a result, the natural-circulation fed DMFC can operate with the fuel delivered in an electrical load-on-demand mode. The feed rates can be optimized for different methanol concentrations by selecting the right tubing size and length of the flow loop. The experimental results also showed that the performance was unstable when the fuel cell operated at low current densities. (c) 2005 Elsevier B.V. All rights reserved.