227Ac is a naturally occurring radioisotope with a unique combination of properties that make it suitable for the determination of deep ocean mixing and upwelling rates. Here, we present a method for the determination of 227Ac in sea water on sample sizes of 20–80 L. The measurement is based on co-precipitation of 227 Ac with MnO2, followed by chemical isolation of actinium in the presence of an artificial Ac isotope. Actinium is then electrodeposited onto silver discs. In two alpha-spectrometric counting periods, first the artificial 225Ac isotope is counted, then after N100 days five daughters of 227Ac. The first counting period gives a total yield for the procedure, integrating chemical recovery and detector efficiency. The total yield was found here to be on average 15±5%, the chemical yield on average about 50%. The counting of five decay products of 227Ac in the second period makes the method particularly sensitive. Using appropriate decay corrections, the initial 227Ac activity can be determined to better than 10% relative error for concentrations b10,000 atoms/L. We compare data acquired by the new method to a data set from in-situ pumps, from a parallel sampling campaign in the Eastern Weddell Gyre, and we can show excellent agreement. Repeated determinations of 227Ac in a uranium reference material (UREM-11) demonstrate the accuracy of the method.