This investigation focuses on low-pressure S2 molecular rf discharges for use as diffuse radiators. Cylindrical fused silica S2-Ar and S2-Ne lamps have been tested at 13.56 MHz and the absolute radiative efficiencies have been measured. The radiation is emitted primarily between 2800 and 3500 Å. The UV radiative efficiencies were studied as a function of input power, buffer gas pressure, discharge radius, and applied external cooling. The maximum observed efficiency for a low power density, diffuse rf discharge is 7%. The measured UV efficiencies of the 13.56 MHz discharges are somewhat lower than those of low-pressure S2-Ar dc discharges. A reentrant lamp design has been tested as well and no improvements in efficiency were observed.