Microstructured flow reactors are powerful tools for the development of multiphase biocatalytic transformations. To expand their current application also to O-2-dependent enzymatic conversions, we have implemented a fully integrated falling film microreactor that provides controllable countercurrent gas-liquid phase contacting in a multi-channel microstructured reaction plate. Advanced non-invasive optical sensing is applied to measure liquid-phase oxygen concentrations in both in-and out-flow as well as directly in the microchannels (width: 600 mu m; depth: 200 mu m). Protein-surface interactions are designed for direct immobilization of catalyst on microchannel walls. Target enzyme (here: D-amino acid oxidase) is fused to the positively charged mini-protein Z(basic2) and the channel surface contains a negatively charged gamma-Al2O3 wash-coat layer. Non-covalent wall attachment of the chimeric Z(basic2)_oxidase resulted in fully reversible enzyme immobilization with fairly uniform surface coverage and near complete retention of biological activity. The falling film at different gas and liquid flow rates as well as reactor inclination angles was shown to be mostly wavy laminar. The calculated film thickness was in the range 0.5-1.3 x 10(-4) m. Direct O-2 concentration measurements at the channel surface demonstrated that the liquid side mass transfer coefficient (K-L) for O-2 governed the overall gas/liquid/solid mass transfer and that the O-2 transfer rate (>= 0.75mM . s(-1)) vastly exceeded the maximum enzymatic reaction rate in a wide range of conditions. A value of 7.5 (+/- 0.5) s(-1) was determined for the overall mass transfer coefficient KLa, comprising a K-L of about 7 x 10(-5) m . s(-1) and a specific surface area of up to 10(5) m(-1). (C) 2016 Wiley Periodicals, Inc.