The CASSCF and DFT methods have been used to determine geometric and electronic structures of the benzoic acid monomer in the S-0, S-1, S-2, T-1, and T-2 electronic states. The S-1/T-2/T-1 three-surface intersection was found by the state-averaged CASSCF calculations, which, in combination with features of the five lowest electronic states, provides new insights into photophysical processes of the benzoic acid monomer. The potential energy profiles of the alpha C-C and C-O bond fissions as well as decarboxylation reaction in different electronic states have been determined for the benzoic acid monomer. The cc C-O bond cleavage starts from the T2 state and leads to the fragments of C6H5CO((X) over tilde (2)A') and OH((H) over tilde (II)-I-2) in the ground state, which is predicted to be the most possible channel upon photoexcitation of the benzoic acid monomer at 270 nm or shorter wavelengths.