An electrodeless lamp waveguide structure (20) includes tuned absorbers (102, 200) for spurious RF signals. A lamp waveguide (24) with an integral frequency selective attenuation includes resonant absorber (102, 200) positioned within the waveguide (24) to absorb spurious out-of-band RF energy. The absorbers (102, 200) have a negligible effect on energy at the selected frequency used to excite plasma in the lamp. In a first embodiment, one or more thin slabs (80, 82) of lossy magnetic material are affixed to the sidewalls (86, 88) of the waveguide (24) at approximately one quarter wavelength of the spurious signal from an end wall of the waveguide. The positioning of the lossy material optimizes absorption of power from the spurious signal. In a second embodiment, one or more thin slabs (80, 82) of lossy magnetic material are used in conjunction with band rejection waveguide filter elements (100, 108). In a third embodiment, one or more microstrip filter elements (200, 202) are tuned to the frequency of the spurious signal and positioned within the waveguide (24) to couple and absorb the spurious signal's energy. All three embodiments absorb negligible energy at the selected frequency and so do not significantly diminish the energy efficiency of the lamp.