In the Martian ionosphere the dominant solar ionization products are O+ and CO2+. These ions are rapidly converted to O2+ by ion neutral reactions resulting in O2+ as the dominant ion. As O2+ has a lower ionization potential, each reaction results in approximately 1.2 eV of energy to be shared by the reaction products. The kinetic energy given to the O2+ will affect the ion temperatures. Calculations have been made of the ion heating rates and temperatures which result from the degradation of these energetic ions for various energy production distributions for conditions similar to those encountered by Viking 1. It is shown that the thermalization of the energetic O2+ can greatly increase the ion temperatures above 200 km compared to those calculated using only the ambient electron heating source. The effect of small horizontal magnetic fields, as predicted by current solar wind interaction models on the ion thermal balance was also investigated. These fields act to restrict the ion thermal conductivity and thus also increase the upper altitude ion temperatures. The combination of the heating by the energetic O2+ and the effect of the magnetic field provide a partial agreement with the Viking 1 measurements.