AbstractA chemical reaction between the building block antiatomic nucleus, the antiproton (p̄ or ${\bar H^ - }$ in chemical notation), and the hydrogen molecular ion (${H_2^ + }$) has been observed by the ATHENA collaboration at CERN. The charged pair interact via the long‐range Coulomb force in the environment of a Penning trap which is purpose‐built to observe antiproton interactions. The net result of the very low energy collision of the pair is the creation of an antiproton–proton bound state, known as protonium (Pn), together with the liberation of a hydrogen atom. The Pn is formed in a highly excited, metastable, state with a lifetime against annihilation of around 1 μs. Effects are observed related to the temperature of the ${H_2^ + }$ prior to the interaction, and this is discussed herein.