Principles of investigating the mechanism of photobreaking of chemical bonds in complicated linear-chain or ring molecules are formulated by the quantum chemistry methods. These methods are then used to investigate photolysis of disulfides, chloronaphthalene, diphenyloxazole, thrinitrotoluene, and coumarine. Mechanisms of primary reactions are established. Electron excited states are determined in which the breaking of a chemical bond occurs as well as the possibility of occupation of these states.