The electronic spectrum and photochemical reaction intermediates of octakis(1,1,2-trimethylpropyl)octasilacubane have been studied by analyzing the UV-VIS spectrum and transient absorption spectra. The first three absorption bands of the UV-VIS spectrum have been assigned to the transitions from HOMO (t(1u) symmetry) to LUMO (a(2u)), from HOMO-1 (t(1g)) to LUMO, and from HOMO to LUMO + 1 (t(2g)) by making reference to the results obtained by INDO/S-CI calculations. The photochemical reaction intermediate has been elucidated to be the isomer whose decay rate constant is 57 +/- 3 s(-1). The quenching rate constant of the isomer by oxygen is ca. 3 x 10(5) M-1 s(-1).