In order to understand the nature of the two transitions in Rochelle Salt, at − 18°C and +23°C, between which points the crystal is ferroelectric, the structures of the two non-ferroelectric phases have been studied with X-rays, preparatory to neutron examination. Both non-ferroelectric phases are in space group P 2 1 2 1 2. Multifilm W eissenberg data with CuKα , and with specimens encased in pyrex, were obtained at temperatures – 64°C, 0°C and + 35°C. Orthorhombic symmetry was assumed for all three phases, and six (001) projections were computed on X-RAC, employing as coefficients: F −64°C obs − F ÷35°C obs , F −64°C obs − F 0°C obs and F 0°C obs − F +35°C obs ; and F −64°C obs , F 0°C obs and F +35°C obs . Starting points for all phase assignments were the coordinates determined by B eevers and H ughes . Repeated refinement cycles led to R -factors of 10.5% for the − 64°C structure, and 11.6% for the 35°C structure. Shifts in atomic coordinates, and changes in thermal oscillations, are reported for both transition points.