Liquid-liquid slug flow offers the unique characteristics of high heat and mass transfer combined with a narrow residence time distribution in continuous flow and has thus attracted considerable attention in the field of microfluidics. To exploit its advantages in the successful design and operation of micro reactors, a precise understanding of the mass transfer processes is essential. In the present work, the role of the thin continuous liquid film formed on the capillary wall in mass transfer is investigated. Fluorescence microscopy is used to determine the exchange between wall film and continuous phase segments to determine if the film is continuously renewed and can therefore be considered to contribute interfacial area available for mass transfer. The distinct wetting properties of different capillary materials are utilized in the experimental set-up to achieve a reproducible and non-invasive release of tracer. The degree of wall film mass transfer as a function of velocity, interfacial area and wall-film thickness is established. (C) 2016 Elsevier Ltd. All rights reserved.