The CAS methods were used for exploring mechanisms of the Cl + N-3 (X(2)Pi) --> NCl((1)Delta) + N-2 (i) and Cl + N-3 (X(2)Pi) --> NCl(X(3)Sigma(-)) + N-2 (ii) reactions. The CASSCF/cc- pVTZ path calculations indicate that channel (i) occurs in the (1)A' potential energy surface (PES) and there exists an intermediate (IM((1)A')) followed by a transition state along the reaction path and that channel (ii) occurs in the (3)A" PES and it has a single step with a transition state. The CASPT2/cc-pVTZ energetic results indicate that channel (i) is feasible while channel (ii) is unfeasible. Preliminary exploration for the (1)A'-(3)A" PES crossing at the CASSCF/cc-pVTZ level implies that the crossing may not cause predissociation of IM((1)A') into the ground state product (NCl ((3)Sigma(-)) + N-2). (C) 2003 Elsevier Science B.V. All rights reserved.