The average oxidation number (O.N.) of S in S_{2}O_{3}^{2-} is +2. Being a stronger oxidising agent than I_{2}, Br_{2 }oxidises , in which the O.N. of S is +6. However, I_{2} is a weak oxidising agent.

Therefore, it oxidises , in which the average O.N. of S is only +2.5. As a result , S_{2}O3^{2-} reacts differently with iodine and bromine.