A catalytic reaction is one where a chemical is needed for the reaction to take place, but the chemical is not affected by the reaction. The simplest catalytic cycle which destroys ozone involves chlorine (Cl) and has chlorine monoxide (ClO) as an intermediary:

Cl + O₃ -> ClO + O₂
ClO + O -> Cl + O₂
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Net Cl + O₃ + O -> Cl + 2O₂

The same amount of chlorine is present before and after the reaction. The Antarctic ozone hole forms when more complex variations of this reaction take place during the early Spring. Bromine (Br), a gas similar to chlorine, can also take part in the catalytic cycles.