(1R(*),2S(*),5R(*))-5-tert-Butyl-2-hydroxycyclopentanecarboxylic acid exists in several solid phases, two of which are investigated in this report : a first phase (phase I) obtained by crystallization from common solvents and a second phase (phase II) obtained by heating the first phase above ca. 70 degrees C. The second phase appears to be the stable phase at ambient temperature, and the first phase appears to be a metastable phase. A third phase (phase III), which reverts to phase II on standing at room temperature, is obtained by cooling the melt to room temperature. The rates of rotation of the tert-butyl groups in phases I and II have been measured by CP/MAS NMR techniques. The activation parameters for the tert-butyl group rotation differ markedly between the two phases : phase I, Delta H not equal = 59.0 +/- 2.1 kJ mol(-1), Delta S not equal = 51.6 +/- 8.1 J K-1 mol(-1); and phase II, Delta H not equal = 43.9 +/- 2.5 kJ mol(-1), Delta S not equal = 28.3 +/- 9.3 J K-1 mol(-1). The structure of phase I has been solved by X-ray diffraction techniques, and this assists in understanding the experimental observations.