Visible chemiluminescence in the spectral region 5500-7300 Å due to the Ba + N2O and Ba + O3 reactions is observed in argon matrices at temperatures below 15 K. Emission is recorded both as thermoluminescence and during deposition. Two molecular progressions of BaO, with ν00 = 17 035 and 17 195 cm-1, and ω″e = 657 and 651 cm-1, are found and are tentatively attributed to emission in the ā 3Σ + → X̃ 1Σ+ transition for BaO* in different matrix sites, though the Ã 1Σ + → X̃ 1Σ+ transition could also be involved. The presumed excitation mechanism is the reactive population of one or both of the singlet states Ã 1Σ+ and/or Ã′ 1Π followed by rapid intersystem crossing to the ā 3Σ+ state. This is the first observation of a chemiluminescent atom transfer reaction in low temperature matrices. Emission due to Ba atoms in the (6s6p1P)→(6s 2 1S) transition is also observed, apparently excited by energy transfer from BaO*, most probably from the Ã′ 1Π state.