Many oxidation processes require the presence of molecular oxygen in the reaction media. Reactors are needed that provide favorable conditions for the mass transfer between the gas and the liquid phase. In this study, two recent key technologies, microreactor technology and biotechnology, were combined to present an interesting alternative to conventional methods and open up excellent possibilities to intensify chemical processes in the field of fine chemicals. An enzyme-catalyzed gas/liquid phase reaction in a falling-film microreactor (FFMR) was examined for the first time. The test reaction was the oxidation of β-D-glucose to gluconic acid catalyzed by glucose oxidase (GOx). Various factors influencing the biotransformation, such as oxygen supply, temperature, enzyme concentration, and reaction time were investigated and compared to those in conventional batch systems. The most critical factor, the volumetric mass-transfer coefficient for the efficient use of oxygen-dependent enzymes, was determined by using the integrated online detection of dissolved oxygen in all systems. The extremely large surface-to-volume ratio of the FFMR facilitated the contact between the enzyme solution and the gaseous substrate. Hence, in a continuous bubble-free FFMR system with a residence time of 25 seconds, a final conversion of up to 50 % in enzymatic oxidation was reached, whereas conversion in a conventional bubble column resulted in only 27 %. Finally, an option for scale-up was shown through an enlarged version of the FFMR.