Abstract: The concentration of boron dissolved in water is determined by positioning a measuring vessel between a radio-active neutron source and a neutron detector. The neutron source is disposed in a moderator and the detector is disposed in a relfector; while the test specimen, e.g. boric acid, is disposed between two flat parallel plates perpendicular to the path between the source and the detector. The time necessary for the detector to receive a certain number of neutrons varies linearly as the concentration of boron; and so the boron concentration can be displayed as a function of elapsed time in excess of that time necessary for the predetermined number of neutrons to be received through pure water.