The mechanism of hydrogen release from solid state ammonia borane (AB) has been investigated via in situ solid state ¹¹B and ¹¹B{¹H} MAS-NMR techniques in external fields of 7.1 T and 18.8 T at a decomposition temperature of 88 °C, well below the reported melting point. The decomposition of AB is well described by an induction, nucleation and growth mechanistic pathway. During the induction period, little hydrogen is released from AB; however, a new species identified as a mobile phase of AB is observed in the ¹¹B NMR spectra. Subsequent to induction, at reaction times when hydrogen is initially being released, three additional species are observed: the diammoniate of diborane (DADB), [(NH₃)₂BH₂]⁺[BH₄]⁻, and two BH₂N₂ species believed to be the linear (NH₃BH₂NH₂BH₃) and cyclic dimer (NH₂BH₂)₂ of aminoborane. At longer reaction times the sharper features are replaced by broad, structureless peaks of a complex polymeric aminoborane (PAB) containing both BH₂N₂ and BHN₃ species. The following mechanistic model for the induction, nucleation and growth for AB decomposition leading to formation of hydrogen is proposed: (i) an induction period that yields a mobile phase of AB caused by disruption of the dihydrogen bonds; (ii) nucleation that yields reactive DADB from the mobile AB; and (iii) growth that includes a bimolecular reaction between DADB and AB to release the stored hydrogen.