The influence of a Surface-active substance upon the behaviour observed for the gas-liquid interfacial area in a bubble column has been studied employing two different techniques: chemical and photographic methods. Important differences between the interfacial area values have been found. and they have been assigned to different phenomena existing in this kind of systems. especially the Marangoni and barrier effect. The experimental results obtained in this work under different operational conditions imply that the chemical method is greatly influenced by the existence of surface tension gradients, and this phenomenon produces high values for the interfacial area. (c) 2008 Elsevier B.V. All rights reserved.