Achieving minimal levels of nitrogen oxides (NO x) during combustion is a major constraint in the design of advanced high-efficiency engines. NO x can be formed during combustion of any fuel—including those without fuel-bound nitrogen—in air, where radicals can attack molecular nitrogen (N 2) present in air to break the strong Nsingle bondN bond to ultimately form NO x. Paramount to the goal of minimizing NO x formation is knowledge of the fundamental routes by which the strong Nsingle bondN bond in N 2 can be broken. Historically, there have been four known routes for breaking the strong Nsingle bondN bond in N 2 to ultimately form NO x. We have recently posited that another route—mediated by an HNNO intermediate—may also play a role, particularly at the high pressures and low peak temperatures relevant to high-efficiency, low-NO x engines. Our previous theoretical and modeling studies show HNNO to be a major product of the N 2 O+ H reaction at high pressures and low temperatures; once formed, HNNO is likely to react with radicals in barrierless reactions that would occur quickly and with high NO x yields. In the present paper, we report measurements of H 2, O 2, H 2 O, N 2 O, NO, NO x, and NH 3 in jet-stirred reactor experiments for an H 2/O 2/N 2 O/NO/N 2/Ar mixture that specifically target HNNO pathways. Importantly, we observe significant formation of NO and NH 3—both of which provide signatures of the HNNO mechanism that are not predicted by previous models without it. Flame simulations using a new sub-model describing pressure-dependent formation and consumption of HNNO show these pathways to be among the most prominent formation routes at high pressures and low peak temperatures. However, exact quantification of the role of HNNO in NO x formation and quantitative predictions of NO x in general require more accurate rate constants for both HNNO pathways and mixture rules for pressure-dependent reactions.