We present a differential retarding potential (DRP) method for improving the kinetic energy resolution of a reactant ion beam for scattering experiments. This method allows ion‐molecule reaction absolute total cross‐section measurements to be performed down to thermal energies using the simple electrostatic aperture ion lenses of a tandem quadrupole mass spectrometric ion‐molecule reaction apparatus, even though the reactant ions are formed originally with a broad kinetic energy distribution. To illustrate the principle of the DRP method, examples are given for its application to reactant ion beams prepared in an electron impact ion source and in an ion‐molecule reaction ion source. ©1994 American Institute of Physics.