1A chlorine oxide (dichlorine monoxide), Cl₂O, an unstable yellow or orangish gas, used in chemical synthesis as a chlorinating and oxidizing agent (also called hypochlorous anhydride) (now rare).

2A free radical, ClO·, formed especially in the upper atmosphere by the action of ultraviolet light on chlorofluorocarbons (CFCs) and contributing to ozone depletion.