The substituent effect on the dynamics of deactivation of the first singlet excited state of para-substituted salicylideneaniline in octanol was investigated using multifrequency phase and modulation fluorometry. On the basis of the temperature dependence and by use of the global analysis approach to the excited-state reaction scheme, we have determined the rate and the activation energy for the cis-keto* --> twist-keto* process. It has been found that the activation energy increases when the electron-donor strength of the substituent increases.