Hydrogen cyanide (HCN) has been the focus of numerous prebiotic chemistry studies since it is hypothesized to be crucial to the abiotic synthesis of sugars and proteins (Xu et al., 2018), and it has been detected in both the interstellar medium (Snyder and Buhl, 1974) and Titan’s atmosphere (Magee et al., 2009). Characterizing the abiotic formation of HCN is thus important for understanding prebiotic chemistry and the origins of life, as HCN deposition and the availability of fixed nitrogen are sensitive to HCN production. Christensen et al. (2024) added three novel HCN-forming reactions to an early Earth photochemical model and found a 5x enhancement in rainout rates, emphasizing the need to deduce these reaction pathways. Here, we characterized HCN formation from far-UV photolysis of CH4 in N2. We simultaneously quantified HCN produced with mass spectrometry and CH4 consumed with cavity ringdown spectroscopy, calculating yields of HCN produced from CH4 photolyzed. Across all experimental conditions, HCN yields were remarkably consistent, falling between 2-3%. To our knowledge, this represents the first experimental effort to account for yields of HCN with respect to CH4 photolyzed. To evaluate if known and/or previously hypothesized HCN production mechanisms could explain our measured HCN values, we used a simple kinetic model for our results, incorporating both experimentally measured and theoretically calculated reactions. Theoretical studies on HCN formation have focused on methylidyne (·CH) radicals and singlet methylene (1CH2) – both photoproducts of CH4 – and their association reactions with N2. When adding theoretical reactions involving ·CH, HCN was either underestimated by 50% using reactions from Christensen et al. (2024) or vastly overestimated using reactions from Saheb (2025). When adding theoretical reactions with 1CH2 from Saheb (2024), HCN was greatly underestimated unless possible but unexplored reactions with proposed intermediates were introduced. To better match our measured HCN mixing ratios, the addition of hypothetical reactions with CH2N2 in the case of networks involving 1CH2 or adjusting proposed rates of HCN2 reactions in the case of ·CH networks, were each required to produce sufficient HCN. Thus, we provide experimental evidence for HCN photochemical formation that is absent from most chemical mechanisms. Our work also suggests future avenues for fundamental investigations. References: Xu, J.; Riston, D.J.; Ranjan, S.; Todd, Z.R.; Sasselov, D.D.; Sutherland, J.D. Chem. Commun. 2018, 54, 5566-5569. https://doi.org/10.1039/C8CC01499J Snyder, L.E.; Buhl, D. Ap. J. (Letters) 1974, 189, L31. https://adsabs.harvard.edu/doi/10.1086/180664 Magee, B.A.; Waite, J.H.; Mandt, K.E.; Westlake, J.; Bell, J.; Gell, D.A. P&SS 2009, 1895-1916. https://doi.org/10.1016/j.pss.2009.06.016 Christensen, M.; Adams, D.; Wong, M.L.; Dunn, P.; Yung, Y.L. Life 2024, 14(5), 601. https://doi.org/10.3390/life14050601. Saheb, V. ACS Earth Space Chem. 2025, https://doi.org/10.1021/acsearthspacechem.5c00337 Saheb, V. ACS Earth Space Chem. 2024, 8 (12), 2474–2482. https://doi.org/10.1021/acsearthspacechem.4c00213