The 133Cs NMR has been observed in a pseudosingle crystal of CsOH · H20 in all three solid state phases. The 133Cs quadrupole coupling constant varies almost linearly with temperature throughout the three phases (e2qQ/h = 156 ± 1 kHz at 193 K, 101 ± 1 kHz at 295 K, 36.4 ± 0.5 at 410 K). There is also a discontinuous change of (13 ± 2) kHz in the cesium coupling at the monoclinic to Hex ß transition. In contrast to previous neutron scattering experiments, which indicate significant atomic motion in the Hex α phase, the temperature dependence of the proton NMR indicates that proton diffusion occurs in the Hex ß phase.