The authors describe a two-dimensional high-resoln. solid-state NMR methodol. for correlating the dynamics of mol. rearrangements around the crit. points of equil. phase transitions in mol. solids. It combines the techniques of temp.-jump and two-dimensional (2D) NMR spectroscopy. The two spectral dimensions are the isotropic chem. shifts at different sample temps. The technique is illustrated by elucidating the dynamic rearrangement of the C4O4 units of squaric acid (H2C4O4) in relation to the mechanism of its antiferroelec. phase transition at TN .apprx. 373 K. These results clarify the apparent discrepancy between the conclusions derived earlier from NMR, x-ray, and Raman and neutron scattering studies. They were a direct consequence of the significant enhancement in the NMR spectral resoln. through this technique, which thus might prove to be a significant new aid in understanding the mechanisms of phase transitions in mol. solids. [on SciFinder (R)]