Characteristics of air-water two-phase flow patterns in a miniature square channel having a gas permeable sidewall were investigated experimentally. Water was fed into the channel from its entrance, while air was injected uniformly into the channel along the permeable sidewall. This configured two-phase flow problem is encountered in direct feed methanol fuel cells. Flow patterns in both vertical upward and horizontal flows were identified using a high-speed motion analyzer. The visualization shows that the typical flow pattern encountered in the conventional co-current gas-liquid two-phase flow, such as bubbly flow, plug flow, slug flow and annular flow were also observed in the present work. However, unlike the conventional co-current gas-liquid two-phase flow in a channel with gas and liquid uniformly entering from one of its ends, for the flow configuration considered in this work, the stratified flow and wavy flow were not found in horizontal flow. And a so-called "single layer bubbly flow" was found in vertical upward flow, which is characterized by a mono small-gas-bubble layer existing adjacent to the surface of the permeable sidewall with the reminding space occupied by the liquid phase. Four transitional flow patterns such as bubbly-plug flow, bubbly-slug flow, plug-slug flow, and slug-annular flow, were found to exist between the distinct flow patterns. Finally, the flow regime maps for various liquid volumetric fluxes are presented in terms of mass quality versus the volumetric flux of gas phase. (C) 2004 Elsevier Ltd. All rights reserved.