Commercially available dimethyl polysiloxane is shown to be an excellent liquid coolant for the containing walls of high power plasmas (≥300 W/cm3) excited with 2.45‐GHz microwaves. This stable, nontoxic, and nonflammable fluid has very low microwave absorption (tan δ=ϵ″/ϵ′=3.5×10−4) at this frequency. Fortuitously, it also has excellent ultraviolet, visible, and near infrared (λ≤0.8 μm) transmission. The UV transmission over 1 cm shows 2% transmission at 2000 Å, a 30% transmission at 2200 Å, 80% at 2400 Å, and 100% for λ≳2500 Å. These features plus its good physical (viscosity, thermal conductivity, specific heat capacity, heat of vaporization, and density) and electrical (resistivity and breakdown) properties make it very unique as a liquid coolant of UV emitting microwave plasma lamps. The clear coolant’s infrared absorption for λ≥1.0 μm reduces the radiative (heat) emission from the plasma. In addition, dimethyl polysiloxane has a very wide operating temperature range, −73 °C≤T≤260 °C. Particular procedures required to assure its excellent ultraviolet transmission are described along with different geometrical combinations of liquid cooled plasma lamps and reflector housings. This liquid may also be used to cool microwave excited laser tubes.