Two mechanisms of OsIVCl6 2- photolysis were studied by means of quantum chemical calculations in gas and aqueous phases. The difference between these mechanisms is in the nature of the possible Os(iv) key intermediates (KI). According to calculations, the intermediate is an OsIVCl5 - complex of square pyramidal coordination geometry. The calculations do not give an opportunity to make an unambiguous choice between the triplet and quintet multiplicities of OsIVCl5 -. The calculated CASSCF/IMCP-SR1 transition energies for 5OsIVCl5 - are lower than for 3OsIVCl5 -, while the calculated XMC-QDPT2/SBKJC spectra for the triplet state are in better agreement with the experimental absorption spectrum of the KI than for the quintet state.