DFT calculations at the Becke3PW91/631+G(d) level of theory provided optimized geometries, transition states, and wave functions suitable for the study of the reactivity and molecular structure with Atoms-in-molecules (AIM) of phosphite ozonide complexes. These calculations also provided activation energies for the extrusion of singlet oxygen from the ozonides, which occurs in a concerted manner. The molecular species investigated were trimethyl phosphite ozonide (1), triphenyl phosphite ozonide (2), trifluoromethyl phosphite ozonide (3), trifluoroethyl phosphite ozonide (4), 4-ethyl-1-phospha-2,6,7-trioxabicyclo[2.2.2]octane ozonide (5), 1-phospha-2,6,7-trioxabicyclo[2.2.2]octane ozonide (6), 1-phospha-2,8,9-trioxadamantane ozonide (7), and propylene phenyl phosphite ozonide (8). Single-point calculations at the Becke3PW91/6311++G(d,p) level on the geometries obtained from the lower level theory yielded activation energies of 15.1 and 16.4 kcal mol1 for the nonconstrained complexes 1 and 2, respectively. These values differed from the electronegative trifluoro derivatives 3 and 4, which had much higher barriers of 23.5 and 20.8 kcal mol1, respectively. The activation energies of the bicyclic complexes 57 were significantly higher than 1 and 2 and comparable to 3 and 4, ranging from 23 to 26 kcal mol1. An intermediate barrier of 20.5 kcal mol1 was computed for 8. AIMPAC studies showed no direct correlation between the AIM atomic charges on the phosphorus or oxygen atoms of the ozonide ring with the ease of decomposition of 18 to singlet oxygen and the corresponding phosphate.Key words: phosphite ozonide complexes, decomposition, DFT methods, AIM, activation energy.