Infrared transmission spectra of 2NH3 ⋅ H2O, 2NH3 ⋅ D2O, 2ND3 ⋅ H2O, and 2ND3 ⋅ D2O have been measured between 100 and 15 K. Spectra of NH3, ND3, NH2D, ND2H, H2O, D2O, and HOD as dilute or major impurities in the above crystals have also been measured. Information about the site splitting, multiple-site splitting, and unit-cell-group splitting, has been obtained for many modes. The absorption by the N–D stretching modes of type II ammonia shows that the unit cell group changes from D2h to Ci at the transitions near 52 K. The corresponding absorption by the type I ammonia retains D2h selection rules at 15 K. The distortion to Ci is greater perpendicular to the ac plane of the D2h structure than parallel to it. The two types of ammonia and one type of water molecule under D2h each splits into two types under Ci, yielding six sets of nonequivalent molecules below 52 K. The existence of two nonequivalent sets of type II ammonia molecules below 52 K, each of which is free to reorient above the transitions, is clearly the source of the two lambda transitions at 51.6 and 53.3 K. The shifts with temperature and isotopic substitution indicate varied anharmonic effects on the intramolecular stretching modes. Thus, the higher frequency stretching bands of H2O and D2O shift by 15 and 20 cm−1, respectively, between 90 and 15 K, while the lower frequency bands, at 2975 and 2230 cm−1, barely shift and that of 2ND3 ⋅ D2O sharpens to a doublet split by 10 cm−1 with an overall half-width of 25 cm−1. The bending modes of H2O and D2O have been identified at 1634 and 1200 cm−1. Fermi resonance with 2ν2 affects only the lower frequency O–D stretching mode of the C1 D2O molecules; the Fermi resonance parameter W is 68± 3 cm−1, close to that in D2O ice 1h. The N–D stretching mode of dilute NH2D shows a remarkable rearrangement of intensity between 35 and 15 K, which suggests that the energy of the crystal is of the order of 270 J mol−1 lower when the N–D bond has a particular orientation. The bands due to ν4 of ammonia have been identified for the first time. It was noted earlier that the frequency difference between the ν2 modes of the two types of NH3 and ND3 molecules was much larger than expected from the frequencies of isolated NH2D and ND2H. This work shows this extra splitting to be due to intermolecular coupling between the two types of ammonia molecules. Other interesting indications of intermolecular interactions, including examples of combination bands due to simultaneous transitions of ammonia and water, are evident throughout the spectra.