The development of an on-line ammonia preconcentration system based on microdistillation is reported. The combination of microdistillation and flow injection gives rise to an improved analytical technique. This method has good throughput (15-20 samples per hour), requires a small sample volume (6.0 ml) and has a low detection limit (2 μg/l). Microdistillation brings the further advantage of using a pH electrode, non-specific for ammonia, as the sensing device. Evaluation of the method showed excellent correlation with the standard phenate method, and good precision for ammonia determination.