Abstract A technique has been developed for in-line monitoring of D 2O concentration in process streams. Commercially available neutron counters are used to detect neutrons from deuterium in water irradiated with 2.62 MeV gamma rays [ 1 2D(γ,n) 1 1H]. Using this technique, deuterium concentrations from 0.015% to 0.13% D 2O in water were measured in laboratory tests with an accuracy of 1% in 30 min. A source-detector configuration was developed and the proposed system successfully tested in the laboratory. The technique was not applied in the existing heavy water plant because of a shortage of funds, and because justification for modification of existing towers was insufficient. Use of the technique for new plants, where towers could be designed initially to include such an instrument, should be much more economically advantageous.