C-13 CP/MAS NMR spectra of the free acid of phenoxymethyl penicillin (penicillin V) and of its lithium, sodium, rubidium, and cesium salts (C16H17N2O5SM, M = H, Li . H2O, Na, Rb, and Cs) have been studied at a range of temperatures between 180 and 400 K. The C-13 CP/MAS NMR spectra at ambient temperature indicate that the structures have one, two, one, four, and one molecule(s), respectively, in the crystallographic asymmetric unit. This is confirmed by the known crystal structure of the free acid and the single-crystal X-ray diffraction studies reported here of the various salts. The variable-temperature C-13 CP/MAS NMR spectra indicate that the phenyl rings of all the molecules perform 180 degrees flips about their local C-2 axes, with measured or extrapolated rate constants in the range 5 x 10(-12) to 8 x 10(9) s(-1) in the temperature regime 200-380 K, and spanning 11 orders of magnitude at ambient temperature. The atomic displacement parameters of the X-ray structural model suggest that these rings also undergo significant in-plane librations. The Arrhenius activation parameters for the ring flip motions have been determined, by analysis of exchange and dipolar broadened NMR line shapes and with 1-D (and 2-D) magnetization transfer experiments, for the free acid and all four salts. The activation barriers have been interpreted in terms of the degree of entrapment of the phenyl groups and the nature of the packing interactions in the different crystal structures.