We investigated the symmetry breaking mechanism in cubic octa-tert-butyl silsesquioxane and octachloro silsesquioxane monocations (Si8O12(C(CH3)(3))(8)(+) and Si8O12Cl8+) using density functional theory (DFT) and group theory. Under Oh symmetry, these ions possess T-2(2g) and E-2(g) electronic states and undergo different symmetry breaking mechanisms. The ground states of Si8O12(C(CH3)(3))(8)(+) and Si8O12Cl8+ belong to the C-3v and D-4h point groups and are characterized by Jahn-Teller stabilization energies of 3959 and 1328 cm(-1), respectively, at the B3LYP/def2-SVP level of theory. The symmetry distortion mechanism in Si8O12Cl8+ is Jahn-Teller type, whereas in Si8O12(C(CH3)(3))(8)(+) the distortion is a combination of both Jahn-Teller and pseudo-Jahn-Teller effects. The distortion force acting in Si8O12(C(CH3)(3))(8)(+) is mainly localized on one Si(tert-butyl) group, while in Si8O12Cl8+ it is distributed over the oxygen atoms. The main distortion forces acting on the Si8O12 core arise from the coupling between the electronic state and the vibrational modes, identified as 9t(2g) + 1e(g) + 3a(2u) for the Si8O12(C(CH3)(3))(8)(+) and 1e(g) + 2e(g) for Si8O12Cl8+.