Ammonia is used as precursor source of hydrogen fuel in an on-vehicle internal combustion engine. Ammonia is stored as, for example, a ligand in an on-vehicle transition metal composition. Upon demand for hydrogen by the vehicle's engine control system, ammonia is expelled as a gas from some of the composition and the ammonia gas is dissociated into a mixture of hydrogen and nitrogen and delivered as a fuel-containing mixture to the engine. In a preferred embodiment, the hydrogen is used as a supplement to gasoline as a fuel for engine operation.