Polymorphism and phase transitions in 2-(2,4-dinitrobenzyl)-3-methylpyridine were investigated using C-13 and N-15 solid-state NMR, crystallographic, and calorimetric techniques. The coexistence of two phases, A and B, over at least 8-9 K around the first-order phase transition temperature is observed by solid-state NMR and by following the decay kinetics of a thermally activated tautomerization process in a single crystal. The observations are interpreted in terms of a distribution of mesoscopic domains within the crystal, differing in their local pressure. The presence of dynamic processes on time scales shorter than minutes is excluded by both NMR measurements and tautomerization kinetics.