CCSD(T)/CBS calculations plus corrections predict the heats of formation of formaldehyde O-oxide 1 and dioxirane 2: Delta H-f(0)(CH2OO, 1) = 28.1 and 26.4 kcal/mol, and Delta H-f(0)(CH2O2, 2) = 3.0 and 1.2 kcal/mol at 0 K and 298 K, respectively. The adiabatic ionization energies are IEa(1) 9.98 eV and IEa(2) = 10.82 eV. Protonation of carbonyl oxide takes place at terminal oxygen with a proton affinity of PA(1) = 203.5 +/- 1.0 kcal/mol. The vertical triplet state (3)A' of 1 is located at 1.82 eV above the ground state (with errors of +/-1.0 kcal/mol or +/-0.05 eV). The parent Criegee intermediate is unstable with respect to O-O bond cleavage but becomes more stable upon ionization. (C) 2007 Elsevier B.V. All rights reserved.