This is an experimental study on the effect of tube diameter, inclination angle and liquid properties on incipient flooding in inclined small diameter tubes, in gas-liquid counter-current flow. Flooding experiments are conducted with four relatively small i.d. tubes (6, 7, 8 and 9 mm) in the range of inclination angles 30-60degrees and smooth inlet/outlet, i.e. a porous tube segment for the liquid and tapered section for gas entry. The effect of liquid properties is examined by using two different liquids, namely water and kerosene. To facilitate the interpretation of flooding data, the free-flowing liquid layer characteristics are also investigated. The results indicate that under these conditions, the effect of tube i.d. and angle of inclination on incipient flooding is significant, but the influence of liquid physical properties is most pronounced. (C) 2003 Elsevier Ltd. All rights reserved.