Abstract: The kinetics of the thermal decomposition of rneso-(1) and dl-dieihyl-2,3-dicyano-2,3-diphenylsuccinatc(2) in styrene at different temperatures were studied by means of dilatometry. The activation parameters were found to be. E_d=93.2 + 3kJ·mol^-1, △S&^#8800;=-12.7+1 eu for 1; E_d=120.8±3kJ·mol^-1, △S&^#8800;= 1.8 ±1eu for 2. The conclusion is that the dl-isomer is more stable than the meso-isomer on the basis of the rate constant for the decompositions and the half life of the two isomers.An explanation is offered for the fact that the final conversion X_∞ of 1 and 2 at lower temperature is larger than that of 1 and 2 at higher temperature in terms of the comprehensive constant k' of the kinetic chain length.