A solid-state hydrogen storage material comprising ammonia borane (AB) and polyethylene oxide (PEO) has been produced by freeze-drying from aqueous solutions from 0% to 100% AB by mass. The phase mixing behaviour of AB and PEO has been investigated using X-ray diffraction which shows that a new 'intermediate' crystalline phase exists, different from both AB and PEO, as observed in our previous work (Nathanson et al., 2015). It is suggested that hydrogen bonding interactions between the ethereal oxygen atom (-O-) in the PEO backbone and the protic hydrogen atoms attached to the nitrogen atom (N-H) of AB molecules promote the formation of a reaction intermediate, leading to lowered hydrogen release temperatures in the composites, compared to neat AB. PEO also acts to significantly reduce the foaming of AB during hydrogen release. A temperature composition phase diagram has been produced for the AB-PEO system to show the relationship between phase mixing and hydrogen release. (C) 2018 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.