The atmosphere is the only significant reservoir on earth where a large part of chlorine and bromine are present in the form of oxidised and organic molecules. The existence of these compounds, in which chlorine and bromine are present in higher oxidation states than −I is partly due to natural circumstances such as oxidation of chlorides and bromides by ozone or ultraviolet radiation present in the atmosphere. For a large part however chlorine and bromine are also present in the form of man made compounds such as chlorofluorocarbons and related organic chlorine compounds such as carbon tetrachloride, chloroform and dichloromethane, the concentrations of which have decreased considerably since the signing of the Montreal Protocol in 1987. Although most chlorine and bromine isotope studies on organic chlorine compounds (but focussed on their occurrence in soils) were described in Chap. 15 a short summary is given here to show the isotope characteristics of this small but important reservoir and to easily compare it to the other reservoirs.