The ammonium contents of granites, pegmatites and hydrothermally altered rocks from SW England have been measured. Ammonium levels in the granites are generally high compared with those from other regions, averaging 36ppm, and they differ markedly between intrusions. The pegmatites show higher ammonium contents than any other igneous rocks which have yet been investigated. Ammonium contents are strongly enriched in the hydrothermally altered rocks, including greisens and kaolinized granites. There is a good correlation between the average ammonium content of the intrusions in SW England and their initial 87Sr/86Sr ratios and peraluminosity. This relationship supports the hypothesis that the ammonium in the granites is derived from a sedimentary source, either in the magmatic source region or via contamination of the magma.