The average O.N. of S in S₂O₃^2- is + 2 while in S₄O₆^2- it is + 2.5.The O.N. of S in S₄O₆^2- is + 6. Since Br₂ is a stronger oxidizing agent than I₂, it oxidizes S of S₂O₃^2- to a higher oxidation state of + 6 and hence forms SO₄^2- ion. I₂, however, being weaker oxidizing agent oxidizes S of S₂O₃^2- ion to a lower oxidation state of + 2.5 in S₄O₆^2- ion. It is because of this reason that thiosulphate reacts differently with Br₂ and I₂.