The product branching ratios for NO +(X 1∑ +) and NO +(a 3∑ +) produced from the reaction of N + with O 2 have been measured at 298 and 500 K in a selected ion flow tube. Approximately 0.5% of the total products are in NO +(a) at both temperatures, despite the fact that the reaction to form NO +(a) is 0.3 eV exothermic. High-level ab initio calculations of the potential energy surfaces for the N + + O 2 reaction show that the reaction from N +( 3P) + O 2( 3∑ g) reactants starts with an efficient early stage charge transfer to the N( 2D) + O 2+(X 2∏) channel, which gives rise to the O 2+(X 2∏) product and, at the same time, serves as the starting point for all of the reaction channels leading to NO + and O + products. Pathways to produce NO +(a 3∑ +) are found to be less favorable than pathways leading to the major product NO +(X 1∑ +). Production of N( 2D) has implications for the concentration of NO in the mesosphere. © 2006 American Chemical Society.