Abstract: The kinetics and activation parameters of thermal isomerization of the meso-isomer (1) and the dl-isomer(2) of diethyl 2,3-dicyano-2,3-diphenylsuccinate in 1,1,2, 2-tetrachloroethane were determined by means of HPLC. The experimental results showed that dl-isomer(2) was more stable than the meso-isomer(1). This fact was explained from the point of view of configuration and the ability to the formation of space bonds.