The industrial-scale performance of gas-liquid reactors is difficult to control when very rapid or highly exothermic reactions are involved. Microstructured reactors offer new opportunities for these reactions by enabling precise heat management and accurate control of operating conditions. The present study examines experimentally the gas-phase mass-transfer characteristics of a reactor tool for the characterization of gas-liquid reactions: a falling-film microreactor. A well-known chemical test system, the absorption of sulfur dioxide SO2 by sodium hydroxide NaOH is employed to determine the characteristics. The measurements of inlet and outlet concentrations in the gas phase enable the mass-transfer coefficient to be determined. The mass-transfer characteristics, in terms of dimensionless Sherwood number Sh(G), are then related to the hydrodynamic characteristics of the gas phase, through the Reynolds number Re-G, and the physico-chemical properties of the reactional system, through the Schmidt number Sc-G. A strong dependence of dimensionless Sherwood number on gas-phase Reynolds number is observed, probably resulting from the specific features of the geometry of the gas-phase inlet. (C) 2011 Elsevier Ltd. All rights reserved.