The influence of a surfactant additive on the interfacial structure and on the transition from the smooth to the wavy stratified flow regime in inclined pipes is investigated for various gas-liquid flow rates. The effect of a surfactant on the gas-liquid stratified downflow patterns is studied using a dilute aqueous solution of a non-ionic surfactant (Tween (R) 80), which is found to have a dramatic effect on two-phase flow characteristics. A conventional solution having a surface tension similar to that of the surfactant solution is also employed and this verifies that the atypical behavior of the surfactant solution is ascribed to its special chemical structure and not to its low surface tension. Liquid layer thickness time records are acquired using a parallel-wire conductance technique from which the statistics of the layer thickness, as well as wave celerities, are calculated. Laser doppler anemornetry (LDA) is employed to investigate the flow structure in the thin liquid layer both with and without interfacial shear induced by a co-current gas flow. A differential pressure transducer is used for pressure drop measurements in the liquid phase. The interpretation of the results verifies that the "delayed" appearance of the interfacial waves should be associated with the turbulence retardation within the liquid layer and it facilitates the clarification of the mechanisms in which the surfactant interacts with the turbulent structures and causes significant drag reduction. (c) 2006 Elsevier Ltd. All rights reserved.