An analysis of the ratios of the di‐ and tri‐atomic ions of hydrogen, deuterium, and H—D mixtures obtained from a hot‐cathode arc discharge in a magnetic field was used to examine the nature of ionic collision and displacement processes. Ion mobility, and consequently the rate of ion‐molecule collisions, at constant pressure, were controlled by varying the intensity of the magnetic field. Statistical treatment of mass spectrograph data indicates that the formation of H3+is due to the reactionH2++H2→H3++Hand that the ion rather than the molecule undergoes dissociation on collision.