[en] A method is described whereby the symmetry of the ammonium ion in crystals may be determined from the number and relative intensities of the N--D stretching and bending fundamentals of isotropically dilute NH3D+ ions. These modes give rise, at liquid-nitrogen temperature, to sharp bands in the infrared spectrum. The method has been applied to the study of the ammonium ion in a variety of crystalline environments, as exemplified by (NH4)2SnCl6, (NH4)2TiF6, NH4IO4, (NH4)2PdCl4, NH4NO3(V), and (NH4)2(COO)2.H2O