Extensive neutron scattering measurements have been carried out on t-butyl cyanide over a wide temperature range. Crystal structures and the potential for methyl-group rotation have been determined and will be reported elsewhere; this paper is concerned with the whole-molecule motions as determined by incoherent quasielastic and inelastic scattering measurements. One transition only has been observed at 233 K and the whole-molecule reorientational motions determined between 190 and 280 K (m.p. 292 K). At all temperatures the molecules undergo rapid reorientational motion about their C-C-N axes in a 3-fold potential which is strongly temperature dependent. The transition is associated with an order of magnitude decrease in the rotational correlation time and the onset of a slower, strongly cooperative motion involving translational as well as rotational displacements of the molecules. These probably involve relaxations between different local structures each having essentially the monoclinic structure of the low-temperature phase and related by rotations of π/2 resulting in the average tetragonal symmetry of the high-temperature phase. The neutron results lead to a self-consistent reinterpretation of previous n.m.r. data.