In the range 308–90 K single crystals of [Ni(H2O)6](NO3)2·(15‐crown‐5)·2H2O continue to diffract well as they undergo three phase transitions. Structures have been determined at 28 different temperatures; data were collected at six more but could not be refined satisfactorily. The transitions identified structurally are in good agreement with those found by thermal analysis. Phase I (above ca 292 K; I2/m, Z = 2, Z′ = ¼) is disordered because the 15‐crown‐5 molecule is located on a 2/m site. Phase II (ca 292–248 K; P21/m, Z = 2, Z′ = ½) is less disordered. Phase III (ca 248–208 K; B21, Z = 28, Z′ = 7 in a commensurate approximation) is modulated with some regions resembling phase II and the rest resembling the fully ordered phase IV (P21/c, Z = 4, Z′ = 1) that is stable below ca 208 K. The modulated structure could be determined well because crystals flash‐cooled to 90 K usually retain the phase III structure. The unusual phase III may be a consequence of the differing thermal contraction of two types of alternating layers, one composed of neutral 15‐crown‐5 molecules and the other composed of the cations, anions and lattice water molecules, which form a hydrogen‐bonded network. The two kinds of layers are linked by Oether...H2Oaxial—Ni hydrogen bonds.