Light metal nitride phases Li3BN2 and Mg3BN3 have been synthesised as potential hydrogen storage materials. The formation processes using boron nitride and amorphous boron have been compared, showing that boron extracts nitrogen from the Li3N and Mg3N2 reactant phases and not from the nitrogen gas. This rate limiting solid state reaction is the first in a three step reaction sequence to form these ternary nitrides. The sensitivity of the reactions to low levels of impurity oxygen in the flow gas caused us to develop a closed reaction system methodology using nickel foil capsules.