In a series of molecular dynamics runs on 188-molecule clusters of tert-butyl chloride spontaneous transitions from the orientationally disordered tetragonal phase to the ordered monoclinic phase occurred during cooling. Nucleation rates exceeding 10(36) critical nuclei m(-3) s(-1) were determined at 100, 90, and 80 K. These were found to be consistent with a rate 8 orders of magnitude lower measured at 156 K in experiments with much larger clusters, when compared on the basis of the classical theory of nucleation. Interfacial free energies of the tetragonal-monoclinic boundaries derived from the rates were similar to 3.5 mJ/m(2), or approximately 0.3 of the heat of transition of the phase change (per unit area of a molecular layer), just as Turnbull. had observed for a series of freezing transitions. Rotational jump frequencies of molecules from the disordered to the ordered orientation were inferred from rotational diffusion in the tetragonal phase and appeared to be satisfactory in the treatment of nucleation. They were far too low, however, to account for the growth rate of postcritical nuclei. A rationale for this discrepancy is proposed.