The first-order phase transition of a hydrate of disodium cytidine 5'-monophosphate (Na2CMP) proceeds depending on the relative humidity around crystals. The number of crystal water molecules per CMP changes approximately from one to nine in the relative humidity range from. 0 to 90%. Crystal structures of three hydrates, whose approximate hydration numbers are nine (an as-grown crystal formed from an aqueous solution; 1), eight (stable in the high relative humidity range; 2); and six and a half (stable in the middle relative humidity range; 3) were determined by X-ray crystallographic analysis. In the three hydrates, CMP molecular layers are formed normal to the b axis, and between molecular layers, there exists a region of water molecules and sodium ions. With the transition from 2 to 3, cell parameter b becomes less than half the original value. The transformation was described as the sliding of every other molecular layer along the a axis: The origin of the transformation is discussed based on hydration schemes around CMP molecules and sodium ions.