L-Glutamic acid (LOA) exists as two polymorphic solid phases (denoted the a and phases) with different molecular conformations. This paper investigates the contrasting dynamic properties of the alpha and phases of LOA, from measurements of solid state H-2 NMR line shapes and spin-lattice relaxation times for the -ND3+ groups in selectively deuterated samples. For both phases. the H-2 NMR line shapes have been simulated successfully in terms of a three-site 120degrees jump motion of the -ND3+ group, with the jump frequency varying from 1.5 x 10(7) to 1.5 x 10(3) s(-1) within the temperature range 373 to 198 K. The activation energy for the reorientation of the -ND3+ group is higher for the alpha phase [(47+/-2) kJ mol(-1)] than for the beta phase [(34+/-3) kJ mol(-1)]. The H-2 NMR spin-lattice relaxation time data are also consistent with the three-site 120degrees jump motion of the -ND3+ group and are in good agreement with the activation energies [alpha phase, (48+/-1) kJ mol(-1), phase, (139+/-2) kJ mol(-1)] determined from our H-2 NMR line shape studies. The differences in the dynamic properties of the -ND3+ groups between the two polymorphs can be rationalized directly in terms of different local environments in the crystal structures, particularly concerning the different hydrogen bonding geometries involving the -ND3+ groups.