Five methods for measuring ambient ammonium aerosol were studied in a laboratory set-up. The five methods are the standard Low Volume Sampler (LVS) as used in the National Air Quality Monitoring Network, tow Modified Low Volume Samplers, A Filter Pack and an Annular Denuder System (ADS). Generated ammonium sulphate and ammonium nitrate aerosol was supplied in added gaseous ammonia. The ADS showed the best performance but is too labourintensive. In the laboratory, the LVS measured about 10-25% less ammonium than the ADS. However, ambient air measurement might yield differen results.