In this paper, we report an in-depth study of the post-milled 4AB/Mg2NiH4 sample, with a special focus on the promoting mechanism of Mg2NiH4 on the dehydrogenation of AB. A combination of X-ray diffraction (XRD), Fourier transformation infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) characterizations, together with selective isotopic labelling and other designed experiments, revealed that AB and Mg2NiH4 react with each other from the starting phase of the dehydrogenation process, which eventually results in the formation of Mg-Ni-B-N-H complexes. On the other hand, it was found that the reaction between AB and Mg2NiH4 cannot proceed directly, but requires phase transition of normal AB to its mobile phase AB* to occur first. Hence, the promoting mechanism of Mg2NiH4 on the dehydrogenation of AB is attributed to its promoting effect on phase transition of normal AB to AB* under mild conditions and in particular its chemical modification of AB with Mg and Ni. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.