Thianthrene has been synthesized by reacting diphenyl sulfide with AlCl3 in heptane at 98A degrees C for 8 h at a diphenyl sulfide: AlCl3 molar ratio of 2: 1; the thianthrene yield is 58%. The reaction mechanism proposed previously has been confirmed by quantum-chemical calculations; the reaction is characterized by a low negative value of Gibbs free energy, a low heat of reaction, and a high activation energy of the first step. Diphenyl ether and diphenylamine do not enter the test reaction because of low nucleophilicity of the heteroatoms; diphenylamine dimerizes under the reaction conditions to form N,N'-diphenylbenzidine.