Abstract Isomerisation of 2, 4, 6-triphenyl-4H-thiopyran into 2, 4, 6-triphenyl-2H-thiopyran has been studied kinetically and the rate of the reaction was determined in dimethylformamide. The first order rate constant could be expressed in terms of the Arrhenius parameters by the equation kt= 2.85× 10 7 e− 15820±110/RT.