A liquid core waveguide membrane microreactor combining intense light matter interaction for in situ sensing and/or photo activation and excellent gas-liquid mass transfer is presented. Basis is a liquid-filled Teflon AF tube, which provides light transmission within the liquid core and gas permeation through the wall. The study shows that a wide spectral range (UV-vis) with relatively low optical losses is accessible. A working regime preventing gas bubble formation was deduced for semi-batch and in flow operation for gas pressures up to 8bar. Residence time distribution experiments revealed Bodenstein numbers from 21 to 60 in the studied flow range. As example, the methylene blue catalyzed oxidation of D-glucose by O-2 was studied at different pressures, while methylene blue was monitored in situ.