Glow discharge mass spectrometry was used to study the occurrence of argon–oxygen ions in rf‐diode sputter deposition discharges. The results show that (ArO)+and (ArO2)+ions are formed over an extensive range of Ar–O2sputtering gas mixtures. The populations of these ions are independent of whether a ZnO or Au target is used and coupled in a manner that indicates (ArO2)+is formed first then dissociates to form (ArO)+. An associative ionization reaction between an Ar atom in a highly excited long‐lived Rydberg state and ground state O2is proposed to be responsible.