The electronic mechanism of the photodecomposition reaction of MnO4- is investigated. The symmetry-adapted cluster (SAC)/SAC-configuration interaction (SAC-Cl) method is used for calculating the ground and excited states of the permanganate ion along the reaction pathway. Electron correlations are very important for reasonable descriptions of the ground and excited states. The 546- and 311-nm absorption bands, which lead to the photodecomposition reaction, are the dipole-allowed transitions to the 1(1)T(2) and 3(1)T(2) states, respectively. The lowest excited state plays a key role in the reaction. The decomposition leading to the ground state of the products, MnO2-((3) Delta(g)(+)) + O-2((3) Sigma(g)(-)), occurs directly along the lowest-excited-state potential curve. The energy barrier existing in the lowest-excited-state curve is the origin of the strong wavelength dependence of the quantum yield and the slight temperature dependence at the longer wavelength region observed by Zimmerman (J. Chem. Phys. 1955, 23, 825). The photochemical and thermal decomposition reactions are shown to be symmetry allowed and forbidden, respectively. The peroxo complex in the 1(1)A(1) ground state corresponds to the long-lived intermediate observed experimentally. The electronic mechanism proposed by the present ab initio study slightly modifies and confirms the experimental reaction scheme (Scheme 1) given by Lee et al. (J. Am. Chem. Sec. 1987, 109, 3003).