Dimethyl disulfide (Me-SS-Me) and methyl ethyl disulfide (Me-SS-Et) in ground (S(0)) and electronically excited (S(1)) states were studied by configuration interaction methods (RCIS and CASSCF). In both molecules, the nonplanar geometric structure with a C-S-S-C dihedral angle of about 88A degrees transforms into the planar structure upon the adiabatic transition S(0) -> S(1). For Me-SS-Me, the vertical transition energy is 5.28 eV and the energy of the adiabatic transition is 2.44 eV. It was shown that the SS bond is elongated by 0.402 A... in Me-SS-Me and 0.453 A... in Me-SS-Et. Calculations showed that the photolysis of both molecules brings the system via the vertical transition to a state approaching the dissociation threshold of the electronically excited molecules into two thiyl radicals. The structure of the thiyl radicals produced was established. The fragments of the potential energy surface for the interaction of the methylthiyl radical with the ethylene molecule were calculated. The activation energy of this process was found to be 3.8 kcal/mol. After passing through the saddle point, the stable radical H(3)C-S-CH(2)-CH(2) is formed which has a transoid configuration.