Some photochemical processes which may be thought of importance for the emission of the sodium lines in twilight and nightglow are discussed. Na + O2 + M → NaO2 + M is taken to be the most important oxidation process. Lack of information about the rate coefficients involved prevents precise quantitative computations, but it is found that photochemical reactions may explain the formation of the emitting layers and the nightglow intensity. However, no satisfactory interpretation of the large seasonal variation has so far been given. Photochemical reactions, as well as photoionization of sodium, seem insufficient to explain the observations.