The phase transitions within the three phases of potassium nitrate (α-, β-, and γ-KNO3) have been investigated in detail by variable-temperature (VT) 15N magic-angle spinning (MAS) NMR spectroscopy employing 98% 15N-enriched KNO3. The stability of the metastable γ-KNO3 phase is found to be highly influenced by the thermal history of the sample, i.e., by the number of heating cycles into the high-temperature β-KNO3 phase at 160°C. Furthermore, the spectral appearance for the room-temperature α-KNO3 phase is found to change after excursions of the sample into the high-temperature β-KNO3 phase, an observation similar to that made in a recent VT synchrotron X-ray powder diffraction study.