Samples of HVPE-prepared hexagonal gallium nitride GaN were subjected to high temperatures under ammonia in order to induce decomposition. Powder XRD, SEM, and solid-state Ga-69 and Ga-71 MAS NMR spectroscopy were used to characterize changes in structure and morphology. The major changes were found to include GaN sublimation and decomposition to the elements in the gas phase. No significant Knight shift effect was detected by gallium NMR in striking contrast to the behavior observed earlier in a similar study of GaN nanopowders. The latter could now be linked to crystal growth and recrystallization phenomena that operate efficiently in pyrolyzed nanopowders while being absent or negligible in heated polycrystalline HVPE materials. (C) 2010 Elsevier B.V. All rights reserved.