Herein, we present a DFT computational study of the trans-[(OsO2)-O-VI(OH)(4)](2) and [(OsO4)-O-VIII(OH)(n)](n) (n = 1 2 cis) comproportionation reaction mechanism that occurs in a basic aqueous matrix. The reaction pathway where [(OsO4)-O-VIII(OH)] reacts with trans-[(OsO2)-O-VI(OH)(4)](2) via an intermediate mediated concerted electronproton transfer yielded the best agreement with experiment (Delta H-double dagger degrees Delta S-double dagger degrees and Delta(double dagger)G degrees experimental data for the forward reaction are 10.3 +/- 0.5 kcal mol(1) 2.6 +/- 1.6 cal mol(1) K-1 and 11.1 +/- 0.9 kcal mol(1) and for the reverse reaction are 6.7 +/- 1.0 kcal mol(1) 63.6 +/- 3.4 cal mol(1) K-1 and 12.2 +/- 2.0 kcal mol(1) respectively where at the PBE-D3 level for the forward reaction are 11.3 kcal mol(1) 9.8 cal mol(1) K-1 and 14.2 kcal mol(1) and for the reverse reaction are 11.8 kcal mol(1) 80.7 cal mol(1) K-1 and 12.3 kcal mol(1) respectively) and consists of (i) formation of a (singlet spin state) noncovalent adduct [(OsOH)-O-VIII-Os-VI](3) (ii) spin-forbidden concerted electronproton transfer (i-EPT) from the trans-[(OsO2)-O-VI(OH)(4)](2) donor to the OsVIII acceptor to form a second (triplet spin state) noncovalent adduct [(OsOHOsVII)-O-VII](3-) (iii) separation of the Os-VII monomers and finally (iv) interconversion of the separated species to form trans-[(OsO3)-O-VII(OH)(2)](2-) and mer-[(OsO3)-O-VII(OH)(3)](2-) stereoisomer species. i-EPT from Os-VI to the Os-VIII species was found to be the rate-determining step which corroborated the experimental evidence (kinetic isotope effect) that the rate-determining step involves the transfer of a proton.