Oxidation and reduction reactions always occur simultaneously. In an oxidation-reduction reaction, one species undergoes oxidation (loses electrons) while another species undergoes reduction (gains electrons). The species that is oxidized is called the reducing agent, while the species that is reduced is called the oxidizing agent.

For example, in the reaction between hydrogen gas and oxygen gas to form water, hydrogen gas is oxidized (loses electrons) to form hydrogen ions (H+), while oxygen gas is reduced (gains electrons) to form hydroxide ions (OH-). The hydrogen gas is the reducing agent, while the oxygen gas is the oxidizing agent.

In summary, oxidation and reduction always occur simultaneously in an oxidation-reduction reaction.