Two-phase phenomena, i.e. bubble flow in the anode and water flooding in the cathode, are critical to design of high-performance direct methanol fuel cells (DMFC). A 5 cm(2) transparent DMFC has been developed to visualize these phenomena in situ. Two types of membrane-electrode assembly (MEA) based on Nafion(R) 112 were used to investigate effects of backing pore structure and wettability on cell polarization characteristics and two-phase flow dynamics. One employed carbon paper backing material and the other carbon cloth. Experiments were performed under conditions of various methanol feed concentrations. The transparent fuel cell was shown to reach a peak power of 93 mW/cm(2) at 0.3 V, using Toray carbon-paper based MEA under 2 M methanol solution preheated at 85degreesC. For the hydrophobic carbon paper backing, it was observed that CO2 bubbles nucleate at certain locations and form large and discrete bubble slugs in the channel. For the hydrophilic carbon cloth backing, it was shown that bubbles are produced more uniformly and of smaller size. It is thus shown that the anode backing layer of uniform, pore size and more hydrophilicity is preferred for gas management in the anode. Flow visualization of water flooding on the cathode side of DMFC has also been carried out. It is shown that liquid droplets appear more easily on the surface of carbon paper due to its reduced hydrophobicity at elevated temperature. For the single-side ELAT carbon cloth, liquid droplets tend to form in the corner between the current collecting rib and GDL since ELAT is highly hydrophobic and the rib (stainless steel) surface is hydrophilic. (C) 2004 Elsevier B.V. All rights reserved.