Combination reactions “combine” elements to form a chemical compound. As usual, oxidation and reduction occur together.

Equation: 2 H₂ + O₂ → 2 H₂O

The sum of oxidation states in the reactants is equal to that in the products:

0 + 0 → (2)(+1) + (-2)

In this equation, both H₂ and O₂ are the molecular forms of their respective elements, and therefore their oxidation states are 0. The product is H₂O: the oxidation state is -2 for oxygen and +1 for hydrogen.