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 8 bar. 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 O2 was studied at different pressures, while methylene blue was monitored in situ.