The mechanism of production of nitrogen oxides by electrical discharges has been examined. The velocity of shock fronts generated by laboratory scale discharges have been measured and are found to be too slow to raise the air temperature to the ~3000 K necessary for nitrogen fixation by the Zel'dovich mechanism. The freeze-out mixing ratio of NOx in air has been measured directly for low-pressure discharges and is found to be of the order expected from the Zel'dovich mechanism for gas cooling over a timescale far longer than the duration of the shock front. Therefore it is concluded that NOx is formed in the gas in the slowly cooling hot channel region and not in the rapidly cooling shock front. Also, it is argued that NOx formation occurs by a freeze out mechanism due to a rapid drop in temperature, not density as has been suggested. NO2 production is found to be significant, with the 2>/ ratio being strongly dependent on the water content of the air. Discrepancies between previous experimental studies of the 2>/ ratio and the quantity of NOx formed per unit energy (P) are discussed. P is also found to vary with spark gap and ambient pressure. It is thought that these effects may be due to a significant loss of heat from the spark gap to the electrodes. The inclusion of atmospheric levels of N2O, CH4, and CO2 are found to have no measurable effect on the yields of NO or NO2. ¿ American Geophysical Union 1996