The influence of non-ionic surfactant additives on the interfacial structure during both co-current gas-liquid upflow and downflow in slightly inclined pipes is investigated. Experiments are conducted in a 24 mm i.d. pipe for various gas-liquid flow rates and pipe inclination angles (beta = +/- 1 degrees and +/- 3 degrees from the horizontal position). Dilute aqueous solutions of the non-ionic surfactant Triton X-100 are employed, in order to study the effect of the additive on the flow pattern and the liquid layer characteristics (i.e. mean layer thickness, amplitude and frequency of the interfacial waves). In particular, the new data of the liquid layer characteristics indicate how the interfacial structure and the transition between the various flow regimes, during co-current gas-liquid flow in slightly inclined pipes, are affected by the addition of small amounts of the non-ionic surfactant. (C) 2009 Elsevier Ltd. All rights reserved.