C₃H⁺₃ is prominent in the reaction zone of a methane + argon diffusion flame, but it is displaced by NH⁺₄ when ammonia is added to the flame. These results may be explained by considering the equilibria in the two proton-transfer reactions H₃O⁺+ C₃H₂= C₃H⁺₃+ H₂O (A), NH⁺₄+ C₃H₂= C₃H⁺₃+ NH₃. (B). In reaction (A), the proton affinity of C₃H₂ is very much greater than that of H₂O, so that H₃O⁺ is only formed in substantial quantities when the concentration of H₂O is several orders of magnitude greater than that of C₃H₂. In reaction (B), the higher proton affinity of NH₃ ensures that NH⁺₄ dominates the reaction zone. Ion cyclotron resonance (i.c.r.) experiments indicate that the proton affinity of C₃H₂ is more than 65 kJ mol^–1 greater than that of ammonia. The concentration of C₃H₂ in the reaction zone was estimated to be 7 × 10¹² cm^–3, lower than the detection limit of mass spectrometry.