Three spin-orbit states, 12Π1/2, 22Π3/2, and 22Π1/2, of chloroiodine anion (ICl-) formed by low-energy electron attachment in the Franck-Condon region are associated with the dissociative limits of I- (1S0) and Cl (2P3/2) or Cl* (2P1/2) fragments. Within the adiabatic scheme, the presumptive Π-symmetry of the fragment angular distributions is dramatically changed to be the Π-Σ mixing symmetry, due to the significant spin-orbit interaction effect on the electronic state couplings of ICl-. The present experimental approach also enables us to separate the contributions of different electronic states from the mixed states, providing a crucial method for quantitatively evaluating the configuration-interaction wavefunctions.