Torsional potentials of 9-phenylanthracene both in the ground and excited states were determined by using ab initio molecular orbital calculations. Molecular structure in the ground state was optimized at the restricted Hartree-Fock level under the C-2c symmetry. Then, the torsional potentials between the anthracene ring and the phenyl group in the S-0 and S-1 states were calculated by means of multi-reference perturbation theory based on the complete active space (CAS) SCF wave function. The present findings were consistent with the jet-laser experimental results by Barbara et al. (C) 2003 Elsevier Science B.V. All rights reserved.