Free flowing liquid layer characteristics, counter-current gas-liquid two-phase flow and incipient flooding were studied in small diameter inclined tubes (7 and 9mm). Experiments were carried out at various inclination angles from the horizontal (30 degrees, 45 degrees, 60 degrees and 75 degrees), while several liquids covering a wide range of physical properties were employed. Fast video recordings and conductivity probes were used for liquid layer thickness measurements, from which mean layer thickness and its statistical quantities were calculated. The wall shear stress at the tube bottom was also measured using an electrodiffusion technique. The new experimental data confirm previous interpretation that in almost all cases the dominant flooding mechanism is wave growth and upward dragging by the gas phase. Consequently, incipient flooding is strongly affected by the liquid layer characteristics, which in turn are influenced by the liquid properties. New correlations based on dimensionless groups for the prediction of flooding in inclined small diameter tubes are proposed. (c) 2008 Elsevier Ltd. All rights reserved.