In order to precisely control the reaction temperature, the photoreactor is placed in the double-walled beaker which is filled with water (Figure and). The mantle of the beaker is connected to a Peltier thermostat (e.g. Lauda LOOP L 100), which circulates water through the mantle and keeps the water inside the beaker at a constant temperature. The advantage of such a thermostat is that it allows for both cooling (to avoid heating up of the dispersion during irradiation) and heating (if the photocatalytic reaction is to be performed at elevated temperature). In our experience, it has been possible to keep the temperature of the dispersion constant within 1 -2 °C during irradiation with this set-up and the temperature can be varied between 10 -70 °C.