The thermolytic decomposition of ammonia borane (AB) is known to proceed through the polymeric coupling reaction between -BH3 and -NH3 sites of multiple ammonia borane molecules, which results in the release of hydrogen and other by-products, e.g., ammonia, diborane and borazine. The formation of these by-products concomitantly pollutes the hydrogen stream, and therefore, it is necessary to remove these gases from the product stream. In the current work, a cost effective and easy to synthesize support material, aluminium phosphate (AP), is introduced in AB thermolytic decomposition. An in situ MS study reveals that the AB and AP (w/w) loading ratio of (1 × 4) is the most promising as it is able to minimise the dehydrogenation peak temperature by 18.89 °C compared to that of pure AB. Additionally, in the presence of support material, the by-product formation of ammonia is reduced by 70.3%, with a complete suppression in borazine and diborane release. The mechanism behind the by-product suppression of supported AB has been studied through 11B MAS NMR analysis which suggests that the release of hydrogen occurs through an intermolecular homopolar B-B bonding. The thermogravimetric and kinetic study also reveals that in the case of supported AB decomposition, hydrogen release through B-B interaction is much more efficient than a B-N interaction, thus limiting the possibility of autocatalysis during the supported decomposition reaction.