The degree of dissociation of molecular nitrogen in 2450 MHz discharges in dry N₂ and N₂-He mixtures at pressures between 0·9 and 6·0 torr have been measured by titration with nitric oxide. Using an X-band resonant-cavity technique, electron (ion) densities in the discharges have also been measured. It is concluded that atoms originate chiefly from thermal dissociation at a quasi-equilibrium gas temperature in the range 3000-3700°K, the neutral gas having been heated by random collisions with electrons at much higher temperatures. Between 0·5 and 2% of the power coupled to the discharge is used in heating the gas.