Neutron diffraction measurements of the intensity variation of certain sensitive Bragg reflections with radiation dosage, establish that the ferroelectric crystal structure of Rochelle salt is seriously affected by exposure to X-rays and γ-rays. Previously, the remarkable changes in the ferroelectric properties of this crystal had been ascribed almost entirely to macrostructural changes, leading to hindered domain mobility. Also, optical studies have thrown new light on the differences in radiation effects in crystals irradiated within and outside the ferroelectric temperature range. In the first case, crystals irradiated to the point where hysteresis loops are no longer observable not only continue to show domain patterns in polarized light within the normally ferroelectric region, as had already been reported, but also show domains at temperatures well outside of that range. On the other hand, crystals subjected to equivalent dosage at temperatures below the lower Curie point show no domains at all-even in the middle of the normal ferroelectric temperature range. It is suggested that there are two basic types of damage which must be considered. One of these involves local damage centers, distributed more or less uniformly throughout the volume of the crystal, and producing structural effects at the unit cell level similar to those produced by introduction of impurity ions in the lattice. In the other case, previously existing imperfections (such as dislocations, e.g., but perhaps also including domain walls) become greatly reinforced by collection of diffusing damage products. The latter type introduces macroscopic clamping effects which hinder domain mobility and tend to stabilize the crystallographic phase existing during irradiation. © 1963.