Abstract Measurements have been made of intensities of the spectral lines emitted from an Ne-O 2 d.c. discharge with small discharge current (1–4 mA) under the following conditions: gas pressures of 2 and 3 torr and oxygen partial pressures (P 0 2 ) up to 0.1 torr. All of the Ne(I) line intensities observed decrease when O 2 is added. The Ne(I) λ5852 line (1 s 2-2 p 1) has been studied in detail as a representative example. The population density of the 2 p 1 level of neon has been obtained from the intensity measurements as a function of P 0 2 . The energy-distribution function of electrons has been determined using Druyvesteyn's method in order to calculate the population density for a corona model. The high-energy tail of the measured distribution function is markedly reduced when O 2 is added. It is shown that inelastic collisions of electrons with O 2 produce large energy losses for the electrons. These cause a decrease in population density of the 2 p 1 level when O 2 is added. The population density of the 2 p 1 level at a gas pressure of 2 torr is 1.2×10 4 cm -3 in pure neon and 5.2×10 2 cm -3 in an Ne-O 2 mixture (P 0 2 = 0.01 torr). The electron densities and average electron energies are 3.5×10 8 cm -3 and 8.7 eV and 1.7×10 8 cm -3 and 5.3 eV, respectively, for the specified two cases.