Abstract: Process for adjusting the precise energy of an ion beam, generally less than 2 eV, emitted by an ion source into a reaction chamber, and causing ion/molecule reactions therein. The reaction chamber is filled with a calibration gas and the voltage which accelerates the ions between the ion source and reaction chamber is varied until the energy-dependent ratio between the rates of production of the two reaction products attains a pre-set value or until the energy-dependent production rate of a reaction product, the energy-dependence of said production rate showing an extreme value, reaches said extreme value.