The mechanisms causing a shift in the oscillation frequency of an RF-excited CO2 waveguide laser through changes in the discharge excitation power are investigated. Frequency shifts in the range of ±0.5 to 1 MHz per watt of RF input power were measured. These are shown to be consistent with the effects of thermal expansion of the laser gas caused by an increase of the gas temperature due to RF power absorption. It is also shown that the effects of gas dissociation are small but significant, whereas, contrary to earlier suggestions, the effects of electron density fluctuations are negligible. The discharge-induced frequency shift may be used as simple frequency modulator with a frequency deviation of ±30 MHz, although the usefulness of this effect is limited to a bandwidth of about 500 Hz, due to the acoustic resonance of the waveguide channel. However, by independent RF power modulation of a short section of the waveguide, this bandwidth has been increased to 3.5 kHz, with a sensitivity of 0.7 MHz/W and a frequency deviation of ±10 MHz.