6 ClO2 (g) + 6 NaOH (aq) → 5 NaClO3 (aq) + NaCl (aq) + 3 H2O (l)
This is an oxidation-reduction (redox) reaction:
ClIV + 5 e- → Cl-I (reduction)
5 ClIV - 5 e- → 5 ClV (oxidation)
ClO2 is both an oxidizing and a reducing agent (disproportionation (dismutation)).