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CaO(XΣ+) plays a central role in the atmospheric chemistry of meteor-ablated calcium. A series of CaO reactions was studied by the pulsed photodissociation at 193.3 nm of calcium acetyl acetonate [Ca(C5H7O2)2] vapor, producing CaO in an excess of reactant and N2 bath gas. CaO was monitored by time-resolved nonresonant LIF, by pumping the CaO(B1Π − X1Σ+) transition at 385.9 nm and detecting B1Π − A1Σ+ emission at λ > 693 nm. The recombination reactions of CaO with H2O, CO2, and O2 were found to be in the falloff region over the experimental pressure range (2−12 Torr). The data were fitted by RRKM theory combined with ab initio quantum calculations on Ca(OH)2, CaCO3 and CaO3, yielding the following results (180−600 K and 0−103 Torr). For CaO + H2O, log (k rec,0/cm6 molecule-2 s-1) = −23.39 + 1.41 log T − 0.751 log T, k rec, ∞ = 7.02 × 10-10 exp(−38.4/T) cm molecule-1 s-1, F c = 0.31. For CaO + CO2:  log (k rec,0/cm6 molecule-2 s-1) = −36.14 + 9.24 log T − 2.19 log2 T, k rec, ∞ = 7.97 × 10-10 exp(−190/T) cm3 molecule-1 s-1,F c = 0.36. For CaO + O2:  log (k rec,0/cm6 molecule-2 s-1) = −42.19 + 13.15 log T − 2.87 log2 T; k rec, ∞ = 9.90 × 10-10 exp(−195/T) cm3 molecule-1 s-1, F c = 0.43 (F c is the broadening factor). The uncertainty in extrapolating to the mesospheric temperature range (120−250 K) is determined using a Monte Carlo procedure. The reaction between CaO and O3 is fast with a small T dependence:  k(204−318 K) = (5.70 +2.01 - 1.43) × 10-10 exp[(−2.22 ± 0.62) kJ mol-1/RT] cm3 molecule-1 s-1, where the quoted uncertainties are at the 95% confidence level. Finally, the implications of these results for calcium chemistry in the mesosphere are discussed.