The nuclear reactions used in analyses described and the optimal flow conditions are listed in tables confirming the predictions regarding analyses for components at very low concentrations in flowing liquids. It was concluded that the twin- stream, twin-detector arrangement can be replaced by the single cell system provided a suitable neutron flux monitoring system with a response proportional to any flux variations across the irradiation cell was available. The single cell system considerably improves sensitivity of measurement, and for process control where extreme precision is not required its use is preferred. Using portable neutron sources, the flow activation technique shows good application to on-stream analyses in a number of industries involved in environmental control and water pollution.