The possibility of deducing reaction mechanisms from kinetic energy released in the products is discussed for thermal energy ion·molecule reactions. An experimental method based on ICR spectrometry is described, allowing for the first time kinetic energy measurements of ions produced in thermal energy reactions. Application to charge transfer from Ne+ and Ar+ to O 2 is presented. Ne+ produces 0+ ions with 0.55 eV mean KE, corresponding to the 0+(4S') + OeD) dissociation limit of oi, while Ar+ produces 0i' molecular ions with 1 eV mean KE corresponding to ground state ions in rather high vibrational states (V~ 8-10). With Ne+ the results can be accounted for in the framework of a (quasi) resonant charge transfer process while with Ar+ charge transfer involves electronic into kinetic energy transfer.