Abstract: The ammonium content of pegmatites, aplites and granites from Alijó-Sanfins, northern Portugal, averages 162, 109 and 159 ppm respectively. There is no systematic difference in the NH4/K ratio between these rocks, and the variation in their ammonium content is mainly due to variations in the amount of potassium minerals they contain. The K-feldspars have higher NH4+ contents than the muscovites, but lower NH4/K ratios. The preference of NH4+ for micas rather than feldspars is not as great as that shown by rubidium. The ammonium content of pegmatites in general is highly variable, and is largely determined by the nature of the granites with which they are associated.