We present experimental results from a partial simulation of the disturbed nighttime ionosphere at 300°K. N2:O2 mixtures at pressures from 0.1 to 30 torr are contained in a 700-liter chamber and irradiated by a diffuse beam of megavolt electrons. In dry gas, only the ions O2−, O2+, and NO+ are seen with the mass spectrometer at the onset of irradiation. As the electron bombardment continues, these ions interact with beam-generated neutral species to produce a spectrum eventually dominated by NO+, NO2−, and NO3−. From measurements of O2+ production and loss rates as a function of irradiation time, the dominant NO production mechanism is shown to be O2+ + N2 → NO+ + NO with a rate constant ≤1×10−16 cm3/sec, followed by ionic recombination to neutralize the NO+. We measure the following thermal-energy two-body ionic recombination coefficients, in units of 10−8 cm3/sec: α(NO+, NO−3) = (3.4±1.2); α(NO+, NO−2) = (17.5±6.0); and α(O2+, O2−) ≤ (14±5). With water added to the gas, the sequence H3O+(H2O)n (n = 0, 1, 2, 3) becomes important in the positive-ion spectrum.