A novel method has been developed for determining the natural decay series radionuclides (NDS), Pb-210, Bi-210 and Po-210, in seawater by way of state-of-the-art liquid scintillation spectrometry. For Pb-210 analysis, the method makes use of a Pb-212 yield tracer, prepared by ion exchange separation from aged Th(NO3)4. Pi-210 recovery is determined using Bi-207 as the yield tracer, and Po-210 is determined using the conventional Po-208 yield tracer. The limits of detection for this method are 0.32, 0.34 and 0.004 mBq/l for Pb-210, Bi-210 and Po-210 respectively. The analysis can be completed within 10 days, as compared with up to one year for traditional methods. Results are presented for a preliminary study of Pb-210, Bi-210 and Po-210 in the dissolved and particle-bound phases of Irish Sea water.