For gas phase O2 measurements, we are using a simple custom-made glass piece, which consists of a NS14 ground glass joint and two parallel flat sides above the connector (see Figure ). Glued inside of this NS14 cap, there are optical sensor spots for O2 and temperature measurement (the temperature measurements is used to correct the O2 measurement signal) and on the outside there are holders for optical fibers (see Figure ). This cap can then be placed on any NS14 Schlenk flask, giving a gastight system with in situ O2 and temperature measurement capability in the gas phase. For liquid phase measurements, a robust probe oxygen sensor can be mounted in a GL14 Schlenk flask using a PTFE screw joint (e.g. from BOLA), which also gives a gas tight connection (see Figure ). The position of the sensor can be adjusted based on the filling level of the photoreactor to make sure that the sensor tip is immersed in the liquid phase. In the shown configuration, there is only sufficient space for the oxygen probe within the photoreactor. For temperature compensation, a Pt100 temperature sensor must then be placed inside the thermostatted water bath next to the photoreactor.