The effect of small upward inclinations on the transition from stratified to slug flow was studied. Experiments were conducted with air and water at atmospheric pressure, flowing in a pipe with a diameter of 0.0763 m and a length of 23 m, at upward inclinations of 0 degrees, 0.05 degrees, 0.2 degrees, 0.4 degrees and 1.2 degrees. Measurements were made of the location of the gas-liquid interface at several locations along the pipe and of the pressure gradient. A critical gas velocity, U-SGC, is determined below which an equilibrium stratified flow was not observed. The mechanism of slugging for upflows is found to differ from horizontal flows in a region in which the liquid layer in an equilibrium stratified flow cannot be supported by the interfacial shear of the gas phase. Within it, slugs are formed intermittently at the inlet. After slug formation, the depleted liquid layer builds up until the height at the inlet is such that a Kelvin-Helmholtz instability can trigger the formation of a new slug. Slugs carry liquid out of the pipe; the stratified flow that exists between slugs carries liquid in the opposite direction. The critical gas velocity is found to correspond to the maximum U-SG in the reversed flow region. Outside this reversed flow region, the transition to slugging can be predicted by the long wavelength viscous stability analysis, by the Kelvin-Helmholtz analysis, or by the slug stability analysis, but slug stability is the dominant mechanism for the system that was studied. (C) 2001 Elsevier Science Ltd. All rights reserved.