The infrared spectra of the ND-stretching bands of dilute NH 3D+ in NH4ClO4 are presented. The spectra are studied over the temperature range from 3 K to room temperature. At low temperatures, the NH3D+ is preferentially oriented. The extent of the orientation at a given temperature depends on the deuterium concentration and thus on the concentration of NH3D+ ions. As the temperature is raised, the various ND-stretching bands collapse due to a variety of dephasing processes. At low temperature, the dephasing process is phonon-assisted tunneling. As the temperature is raised, dephasing is accomplished by interaction among librational states. At still higher temperatures, the bandwidth is due to stochastic motion of the ammonium ions. © 1983 American Institute of Physics.