We investigate the influence of an internal barrier on an exothermic adiabatic reaction model between diatomic ions and molecules. Reaction cross-sections are calculated from quasi-classical trajectories for different initial vibrational and rotational states of the reactants and for relative collision energies in the range from 0.01 to 3 eV. It is shown that the height of a late internal barrier strongly influences both the characteristics of the state-selected cross-sections and the energy distributions of the products. In contrast to complex formation in the entrance region according to the Langevin model our analysis emphasizes the role of the full potential energy surface for an understanding of the dynamics of ion-molecule reactions.