Proton, deuterium, and sodium-23 nuclear magnetic resonances of deuterated and ordinary lecontite have been studied from 77° to 300°K and particularly in the vicinity of the ferroelectric phase transitions at 92° and 101°K. Proton-proton vector directions were determined for the two kinds of water molecules in the unit cell and are observed not to alter as the phase transitions are traversed. Above 230°K the deuterium magnetic resonance of one type of water molecule is observed in the 180° flip state of motion. Below 101°K axial and equatorial deuterons of ND4+ groups rotating about the threefold axis are observed with no detectable change at the transition temperatures. At 300°K one kind of 23Na site is observed; below 101°K, 23Na satellite line splitting occurs indicating that there are two kinds of 23Na site in the unit cell. This splitting is attributed to an abrupt lattice distortion which occurs as the crystal is cooled below 101°K. Proton spin-lattice relaxation time data show two minima, one at 170°K and one at 120°K, which are, respectively, assigned to rotation about the twofold and threefold axes of the NH 4+ tetrahedron. Rotating frame relaxation times show minima at 80°, 110°, and 210°K, respectively, due to the threefold and twofold axis rotations of the NH4+ ion and a 180° flip motion of one kind of H2O molecule. Near 101°K the activation energy for the threefold axis rotation of the NH4+ ion changes from 2.0 to 3.1 kcal mole-1. It is proposed on the basis of the magnetic resonance data that the transition at 101 °K in lecontite is of the displacive type with a relatively large lattice distortion.