Natural gas sourced from Carboniferous coal-bearing strata is produced from Carboniferous, Permian and Triassic reservoirs in the Southern Permian Basin in northwest Europe. The composition of this gas has been reviewed. In all reservoirs a distinct area rich in non-hydrocarbons, principally nitrogen, coincides with the basin depocentre in northern Germany. Furthermore, the average nitrogen concentration increases upwards in the stratigraphy. Carbon dioxide is a minor component which is geographically restricted and occurs mainly in Carboniferous and Zechstein reservoirs. It is generated indigenously in these reservoirs and does not migrate into other reservoirs. Hydrogen sulphide occurs only significantly in the Zechstein and is areally restricted to the basin margin facies of this reservoir. It is formed by thermochemical sulphate reduction. The maturity of the source rock, timing of the trap formation relative to the gas source maturation and reservoir lithology appear to be the dominant controls on the non-hydrocarbon gas composition in the Southern Permian Basin. For very high nitrogen gases (50-100 vol. %), an admixture of inorganic nitrogen to the thermogenic gas from organic sources cannot be excluded.