Equil. geometries and vibrational frequencies of the ground and some excited states of p-chlorotoluene were calcd. by the complete active space SCF (CASSCF) method. Multi-ref. CASSCF second order perturbation theory (MSCASPT2) calcns. were performed on the vertical excitation energies of six singlet and triplet excited states. The potential energy curves along the Cl-C6H4CH3 bond distance of a no. of low-lying singlet and triplet excited states were calcd. by the CASPT2 method based on CASSCF partially optimized geometries. The fluorescence and one component of the dual phosphorescence obsd. exptl. were clearly explained by the CASPT2 calcd. transition energies. According to those CASPT2 potential energy curves, the photodissocn. of p-chlorotoluene at 266 nm was attributed to the predissocn. of the first triplet excited state after its intersystem crossing with the first singlet excited state. The internal rotation and substitution effects of Me on the photodissocn. were discussed in detail.