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A field intercomparison with automatic ammonia monitors was held in Bilthoven, the Netherlands, for a period of two weeks. Six instruments were continuously and simultaneously operated to measure the ammonia concentration in ambient air. During the intercomparison these instruments were studied with respect to their performance and potential suitability as a monitor for the National Air Quality Monitoring Network (NAQMN) of the Netherlands. The instruments were a continuous-flow denuder, developed by ECN, two thermodenuders, one developed by KEMA and the other by ECN, a commercial DOAS system of OPSIS, a photoacoustic monitor, developed by KEMA, and a (non-automatic) Annular Denuder System, used as a reference. With the latter method ammonium concentrations were also measured. The continuous-flow denuder proved to be a good instrument for accurate and fast measurement of ammonia concentrations, but needs frequent maintenance and calibration. Proper operating of the photoacoustic monitor was impaired by problems with the stability of the CO2 laser. The performance of this monitor could not be well studied due to frequent readjustment of the laser. The detection limit of the DOAS system was high, i.e. 4 mug m-3 based on a 3-min averaging time and 0.9 mug.m-3 based on a 60-min averaging time, whereas 0.5 mug m-3 is required. Its precision was also less than optimal. Both thermodenuders fulfil most of the criteria for use in the NAQMN, but the have to be significantly improved. Their performance will be studied further under controlled conditions.