Flooding experiments have been carried out in vertical small i.d. tubes (6, 7, 8 and 9 mm), using smooth inlet and outlet conditions, with air and two liquids (water and kerosene). Experimental data on free falling film characteristics have also been obtained, in the same test sections, which aid the interpretation of flooding phenomena. These new data suggest that the tube diameter strongly affects film flow development, possibly promoting wave interaction and damping. In turn, the wavy film evolution essentially determines flooding characteristics. At small liquid Reynolds numbers (Re-L < 300) critical flooding velocities, U-G, follow a trend already reported in the literature, i.e. decreasing with increasing liquid rate. However, at higher Re-L the trend is reversed, i.e. increasing U-G with increasing liquid rate. This has not been reported in the literature before and may be attributed to damping of waves. At still higher Re-L, another region is evident in the flooding curves, characterized by nearly constant flooding velocity. The dominant mechanism in almost all cases is wave growth and upward dragging by the gas, initiated at the liquid exit. (C) 2002 Elsevier Science Ltd. All rights reserved.