In sulphur dioxide (SO_{2}), the oxidation number (O.N.) of S is +4 and the range of the O.N. that S can have is from +6 to –2.

Therefore, SO_{2} can act as an oxidising as well as a reducing agent.

In hydrogen peroxide (H_{2}O_{2}), the O.N. of O is –1 and the range of the O.N. that O can have is from 0 to –2. O can sometimes also attain the oxidation numbers +1 and +2.

Hence, H_{2}O_{2} can act as an oxidising as well as a reducing agent. In ozone (O_{3}), the O.N. of O is zero and the range of the O.N. that O can have is from 0 to –2. Therefore, the O.N. of O can only decrease in this case. Hence, O3 acts only as an oxidant.

In nitric acid (HNO_{3}), the O.N. of N is +5 and the range of the O.N. that N can have is from +5 to –3. Therefore, the O.N. of N can only decrease in this case. Hence, HNO_{3 }acts only as an oxidant.