In marine systems, 210Po and 210Pb disequilibria are being increasingly used to examine oceanic particle formation and export. Here, an updated assessment of current methods for determining 210Po and 210Pb activity in marine samples is provided and includes a complete description of the vast number of calculations and uncertainties associated with Po and Pb loss, decay, and ingrowth during sample processing. First, we summarize the current methods for the determination of 210Po and 210Pb activities in dissolved and particulate seawater samples and recommend areas for improvement. Next, we detail the calculations and associated uncertainties using principles of error propagation, while also accounting for radionuclide ingrowth, decay, and recovery. A spreadsheet reporting these calculations is included as a downloadable Web Appendix. Our analysis provides insight into the contributions of the relative uncertainty for each parameter considered in the calculation of final 210Po and 210Pb activities and gives recommendations on how to obtain the most precise final values. For typical experimental conditions in open seawater, we show that our method allows calculating 210Pb activity with a relative uncertainty of about 7%. However for 210Po activities, the final relative uncertainty is more variable and depends on the 210Po/210Pb activity ratio in the initial sample and the time elapsed between sampling and sample processing. The lowest relative uncertainties on 210Po that can be obtained by this method is 6% and can only be obtained for samples with high 210Po/210Pb activity ratios (>1) that were rapidly processed. © 2013, by the American Society of Limnology and Oceanography, Inc.