The chemistry of seven ions, O2+, NO+, CO2+, O+, N2+, CO+, and H+, is studied in the nightside ionosphere of Mars using the analytical yield spectrum approach and coupled continuity equations for the steady state condition. The source and sink processes of these ions are discussed in detail. We have found that the nightside Martian ionosphere produced due to precipitation of magnetotail electrons agrees with the Viking observations rather than other sources of ionization such as the precipitation of plasma sheet electron or horizontal plasma transport from dayside ionosphere to the nightside similar to that operating on Venus. The precipitation of magnetotail electron produces major ion CO2+ which is quickly removed by the reactions (1) CO2++O→O2++CO, (2) CO2++O→O++CO2, and (3) O++CO2→O2++CO, leading to O2+ as a dominant ion in the nightside ionosphere of Mars. The reactions of O2+ with N and NO are the major sources of NO+, which is lost entirely by dissociative recombination reaction. The second reaction is the major source of O+ below 190 km. Above this altitude the magnetotail electron impact ionization process is an important source of O+. This process is also important for the production of CO+ and N2+. The ion O+ is destroyed by the third reaction, while CO+ and N2+ are destroyed by charge transfer with CO2. The ion H+ is produced due to charge exchange reaction between O+ and H which is lost by CO+.¿ 1997 American Geophysical Union