Emission in the ultraviolet from the A 2Σ+ electronic excited states of OH, NeD, and ArD, and the formation kinetics of these excited heteronuclear diatomics, have been investigated in microcavity plasmas generated in rare gas/H2O or D2 gas mixtures. Excitation transfer from the a 3Σu+(1u,0u−) Rydberg state of Ar2 appears to be the dominant pathway to OH(A 2Σ+) formation in Ar/H2O vapor mixtures with total pressures of 400–800 Torr and H2O partial pressures of 100 mTorr–3 Torr. Maximum emission on the (v′,v″) = (0,0) vibrational band of the OH(A→X) transition is observed in a 25 μm, 45 nl microcavity for 600–800 Torr Ar/0.5 Torr H2O mixtures. Comparisons of experimental and simulated fluorescence spectra show the OH[A 2Σ+(v′ = 0)] state rotational temperature to be 425 K for 600 Torr Ar/100 mTorr H2O mixtures but to rise linearly with the H2O partial pressure and exhibit a slope of 170 K/Torr H2O for 100 mTorr ⩽ pH2O ⩽ 3 Torr. Excitation of Ne or Ar/D2 gas mixtures in 50×50 arrays of Si microplasma devices generates broadband spectra, peaking in the mid-ultraviolet (λ ∼ 280–320 nm), which are attributed to the A→X transition of the ArD or NeD excimers. The optimal D2 concentration is observed to be ∼ 0.5% and the primary kinetic formation mechanism for the deuterides involves D atom transfer in collisions between Ar(4s 3P) and D2.