The S = 2 complex, manganese(III) meso-tetra(4-sulfonatophenyl)porphine chloride ((MnTSPP)-T-III) is a highly efficient relaxation agent with respect to water protons and has been studied extensively as a possible MRI contrast agent. The NMR relaxation mechanism has several unique aspects, key among which is the unusual role of zero-field splitting (zfs) interactions and the effect of these interactions on the electron spin dynamics. The principal determinant of the shape of the R, magnetic relaxation dispersion (MRD) profile is the tetragonal 4th-order zfs tensor component, B-4(4), which splits the levels of the m(s) = +/- 2 non -Kramers doublet. When the splitting due to B-4(4) exceeds the Zeeman splitting, the matrix elements of (S, are driven into coherent oscillation, with the result that the NMR paramagnetic relaxation enhancement is suppressed. To confirm the fundamental aspects of this mechanism, proton R, MRD data have been collected on polyacrylamide gel samples in which (MnTSPP)-T-III is reorientationally immobilized. Solute immobilization suppresses time-dependence in the electron spin Hamiltonian that is caused by Brownian motion, simplifying the theoretical analysis. Simultaneous fits of both gel and solution data were achieved using a single set of parameters, all of which were known or tightly constrained from prior experiments except the 4th-order zfs parameter, B-4(4), and the electron spin relaxation times, which were found to differ in the ms = 1 and ms = 2 doublet manifolds. In liquid samples, but not in the gels, the B-4(4) -induced splitting of the ms = 2 non-Kramers doublet is partially collapsed due to Brownian motion. This phenomenon affects the magnitudes of both B-4(4) and electron spin relaxation times in the liquid samples.