State-selected Coulomb-crystallized molecular ions were employed for the first time in ion-molecule reaction studies using the prototypical charge-transfer process N-2(+) + N-2 -> N-2 + N-+(2) as an example. By preparing the reactant ions in a well-defined rovibrational state and localizing them in space by sympathetic cooling to milliKelvin temperatures in an ion trap, state- and energy-controlled reaction experiments with sensitivities on the level of single ions were performed. The experimental results were interpreted with quasi-classical trajectory simulations on a six-dimensional potential-energy surface which provided detailed insight into translation-to-rotation energy transfer occurring during charge transfer between N-2 and N-2(+). (c) 2012 Elsevier B.V. All rights reserved.