This study is an experimental investigation of two-phase air/water flow in small horizontal rectangular channels of the same hydraulic diameter (0.87 cm), but of three different aspect ratios. Flow pattern maps were generated for each of the channels and compared with other maps appearing in the literature. From these comparisons it appeared that neither the channel cross-sectional shape nor the aspect ratio appreciably influenced the transition from slug to annular flow. The plug- slug transition was found to occur at higher gas phase superficial velocities, than predicted from flow regime maps in the literature developed for circular cross-section channels, over a range of superficial liquid velocities. Third, the bubble-plug flow pattern transition appeared to have a small dependency upon aspect ratio; Only qualitative agreement was shown with the transitions of the Troniewski-Ulbrich  and Fukano  studies.