Crystals of hexamminecobalt(III) nitrate are found not to be cubic like those of the thiocyanatopentammine derivative but to be tetragonal and of space group symmetry P42nm. The unit-cell parameters are a = b = 21.66, c = 33"32 + 0"05 A. The most prominent feature of the difference in response to X-rays of these and similar crystals is the heavy diffuse scattering produced by the hexamminecobalt(III) nitrate. By means of a series of cylindrical Laue photographs, taken with monochromatic cobalt (Co Ke) radiation, covering a quadrant of reciprocal space at regular intervals of 5 ° about the tetragonal axis, the diffuse domains in the reciprocal lattice are explored and information obtained on the nature and direction of propagation of the thermal waves in the crystal. An interesting feature of the crystal dynamics of hexamminecobalt(III) nitrate is that the only effects observed as a result of independent motion of large structural units are those of transverse waves. There are no noticeable similar effects ascribable to longitudinal waves. Thus information is obtained on the behaviour, shape and orientation of the structural units [Co(NH3)613 + and (NO3)-.