An optical measuring technique, combining Planar Laser Induced Fluorescence (PLIF) with the effect of fluorescence quenching, is used for the local concentration analysis of a gas absorbed and desorbed in a thin liquid film flow. The absorption and desorption process is visualized by using oxygen in a mixture of water, ethanol and glycerol. Fluorescence is strongly reduced by the presence of molecular oxygen as a result of dynamic quenching. The resulting decrease in intensity is detected and interpreted through image processing. The image analysis provides a spatiotemporal value of fluorescence intensity and therefore enables the direct characterization of local transport phenomena in the liquid film flow. In this way, the impact of a single pyramidal microstructure on mass transfer into the film flow is examined. The results are compared to liquid film flow over a smooth surface. The measuring methodology is validated through diffusion experiments. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.