The n.m.r. parameters T1 and T1ρ, the spin-lattice relaxation times in the laboratory- and rotating-frames, respectively, have been utilized to study the molecular motions that occur in polycrystalline samples of cytosine and cytosine hydrate in the range 130–293 K. The deuterated analogues have also been studied. It is concluded that, in the monohydrate, the water molecule, but not the cytosine itself, may rotate (at a frequency of ca. 30 kHz at 200 K), the activation energy of rotation being 17.8±2.8 kJ mol–1. There is also evidence for the rotation of the amino group. Dehydration of cytosine hydrate commences around 328 K, and the enthalpy of dehydration is estimated to be 50 kJ mol–1.