Ammonia borane (AB) is an attractive hydrogen storage material with high gravimetric and volumetric density of hydrogen. The thermolysis of solid ammonia borane follows sigmoidal kinetics with an "induction period" prior to rapid hydrogen release, generating a complex mixture of spent fuel products. We present evidence for the induction period involving isomerization of AB into the diammoniate of diborane (DADB). The induction period is substantially decreased with the addition of DADB. The induction period is also dependent on AB purity and temperature. Reducing the induction period and enhancing the rate of hydrogen release ultimately makes AB an attractive hydrogen storage material for fuel applications. This study profiles the mechanism of the thermolysis of AB, the effect of AB purity and the effect of chemical additives on the induction period and rates of dehydrogenation of AB.