Techniques have been developed for the crossed-beam study of certain simple reactions as a function of energy. An approximately monoenergetic ion beam is crossed with a thermal molecular beam and ionic products are characterized as to their nature, angular distribution, translational energy and, by difference, internal energy. Such measurements may be made as a function of energy available in the centre-of-mass system in the range 0.1–25 eV. Initial studies using this method have been made on the reactions Ar++D2→ArD++D and N+2+ D2→N2D++ D. Contrary to earlier suggestions, their mechanisms are not dominated by an intermediate of relatively long life. Instead, even at the lowest energies, these reactions appear to be “direct” in that the collision complex has a lifetime comparable to, or less than, one molecular rotation. The results suggest a new general model for direct reactions which postulates dominance by long-range forces. This predicts product energies, and angular distributions well in accord with experimental findings. At lowest reactant energies, part of the energy of reaction appears as translation, but as the incident velocity increases, the situation reverses and there is conversion of translational to internal energy. At highest reactant energies, spectator stripping is approached as a limiting case. Interpretation of the data by this model provides reasonable estimates for the critical internuclear distance in the intermediate state.