A plasma with a volume of 10^-4 m³ and a very low degree of ionization can be maintained inside a high-Q cavity using up to 1.4 kW of microwave power at 2.45 GHz. Various working gases have been employed, including air, nitrogen and noble gases at pressures up to 2 × 10⁵ Pa. The reported results are for nitrogen at atmospheric pressure. The plasma absorbs about 80% of the incident power implying a mean root mean square electric field in the cavity for plasma maintenance of 50 kV m^-1 , which is much smaller than that required for breakdown. Estimates of the mean heavy species temperature from the form of the band spectra at 315 and 340 nm give approximately 2200 K. The electron collision frequency and the ratio E /N are 4 × 10¹¹ s^-1 and 1.5 × 10^-20 V m² , respectively. Since the electron collision rate is much higher than the microwave frequency, previous results for a dc electric field obtained by Engelhardt et al can be used to infer a characteristic electron energy of 1.1 eV, with a strongly non-Maxwellian distribution function, and an electron density of 7 × 10¹⁶ m^-3 . A substantial fraction of the input power is transferred from electrons to the heavy species by inelastic collisions, principally due to the excitation of molecular vibration.