Base liquid film thickness distribution, wave behavior, and pressure loss measurements have been obtained for 237 horizontal two-phase (air-water) flow conditions in 8.8 and 15.1 mm I.D. tubes in the wavy and annular regimes. The behaviors measured indicate the presence of a transitional wavy-annular regime at flow rates traditionally labeled as annular in these small diameter tubes. Data for a 26.3 mm I.D. tube do not show these same trends. (C) 2008 Elsevier Ltd. All rights reserved.