In-house built PC based rapid conductometric titration method for assay of trace amount of boron in light water and heavy water [100 ppb (mu g L-1)] with a precision of 2.2 has been reported. The method involves conversion of trace quantity of extremely poor conducting boric acid present in the sample to a relatively better conducting complexing species boron-mannitol complex using mannitol. This complex is then titrated against NaOH. Minor shift in conductivity during reaction of the complex compound with NaOH is captured online using an indigenously developed high resolution conductometric titration device. In addition, the technique provides information about any reactive chemical specimens, if present in the sample. The entire titration process is completed within a couple of minutes. A high resolution conductivity measurement technique has been proposed for quick assay of boron down to 20 ppb B level (RSD 4.1%). The present technique is suitable for rapid assay of boron in heavy water moderator in pressurised heavy water reactors. The influence of isotopic effect on sensitivity of measurements of boron in light water and heavy water matrices is also described.