This paper presents experimental counter-current air-water flow data on the onset of flooding and slugging, the slug propagation velocity, the predominant slug frequency and the average void fraction collected by using different size orifices installed at two locations in a horizontal pipe. For the flow conditions covered during these experiments, it was observed that there is no significant difference between the onset of flooding and the onset of slugging when an orifice is installed in the horizontal run. However, a difference was observed for the experiments carried out without orifices. Furthermore, the position of the orifice with respect to the elbow does not affect the onset of flooding and slugging. When an orifice is installed in the horizontal run, it was observed that slugs occur due to the mutual interaction (constructive interference) of two waves traveling in opposite directions. This means that a completely different mechanism seems to govern the formation of slugs in counter-current two-phase flows in horizontal partially blocked pipes. This is in contrast to that described for the slugging phenomena in co-current flow, where wave instability seems to be the principal mechanisms responsible of bridging the pipe. The mutual interaction of waves traveling in opposite directions seems to control the behaviour of the slug propagation velocity, the slug frequency and average void fraction with increasing the gas superficial velocity. (c) 2005 Elsevier Ltd. All rights reserved.