# In an oxidation-reduction reaction, what substance is reduced always?

Formally, a species that gains electrons is reduced, and a species that loses electrons is oxidized. When oxygen (typically) is reacted with a hydrocarbon fuel, the oxygen is formally reduced to $C {O}_{2}$, in which the oxygen number of $O$ is $- I I$, REDUCED from the zerovalent elemental state. On the other hand, the $C$ has been oxidized from $- I I$ or so to $+ I V$.