Ammonium sulfate (AS) particles are widely used for studying the physical–chemistry processes of aerosols and for instrument calibrations. Small quantities of organic matter can greatly influence the studied properties, as observed by many laboratory studies. In this work, monodisperse particles (200–500 nm aerodynamic diameter) were generated by nebulizing various AS solutions and organic impurities were quantified relative to sulfate using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The organic content found in AS solutions was also tentatively identified using a liquid chromatography–tandem mass spectrometer (LC–MS). The results from both analytical techniques were consistent and demonstrated that the organic impurities contained oxygen, nitrogen, and/or sulfur, their molecular masses ranged from m/z 69 to 420, and they likely originate from the commercial AS crystals. For AS particle sizes ranging from 200 to 500 nm, the total mass fraction of organic compounds (relative to sulfate) ranged from 3.8 % to 1.5 %, respectively. An inorganic–organic mixture model suggested that the organic impurities were coated on the AS particle with a surface density of 1.1 × 10−3 g m−2. A series of tests were performed to remove the organic content (using pure N2 in the flow, ultrapure water in the solutions, and very high AS quality), showing that at least 40 % of the organic impurities could be removed. In conclusion, it is recommended to use AS seeds with caution, especially when small particles are used, in terms of AS purity and water purity when aqueous solutions are used for atomization.