Sustainably produced ammonia is attracting significant attention as a carbon-free fuel with established production methods and distribution logistics. Serious disadvantages include exceptionally low chemical reactivity and potentially unacceptable emissions of oxides of nitrogen. The former can be mitigated through blending with substantial amounts of hydrogen. A potential alternative is to enhance the reactivity of ammonia through the use of hydrazine. The latter is exceptionally reactive with a long history of use in industrial and high-performance propulsion applications. The chemistries of hydrazine-ammonia are also interdependent to a significant extent through common reaction pathways. However, some of these are not well-defined quantitatively and the current work initially assesses recent progress leading to a validated detailed chemistry model based on fundamental rate parameters. The model is shown to be capable of reproducing measured trends sufficiently well to form the basis for an assessment of the potential of ammonia-hydrazine fuel blends. The applicability is explored computationally under conditions of relevance to industrial- and aero-engines and it shown that the addition of small to moderate ( \(\le 20\) %) amounts of hydrazine leads to large reactivity increases and a two-stage ignition behaviour. The implications for hazard assessments and potential emissions of nitrous oxide, in addition to nitric oxide, under high pressure conditions are also highlighted. It is further concluded that the complex chemistry of ammonia/hydrazine fuels must, due to the wide range chemical timescales, be combined with accurate computational methods for turbulence-chemistry interactions to provide meaningful design tools.