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Concentrations of the naturally occurring radionuclides 210 Pb and 210 Po were determined in both the dissolved (<0.2 μm) and particulate (>0.2 μm) fractions in surface waters of the northwestern Mediterranean Sea. About 4-8% of the 210 Pb activity was observed to be associated to particles, which were determined as being mostly biogenic. The 210 Po content in the particulate fraction was generally greater than that of 210 Pb, accounting for about 20% of the total activity. Total activities of 210 Pb and 210 Po were found to be in equilibrium. 226 Ra concentrations were also determined in an effort to investigate its occurrence in Mediterranean waters. 226 Ra was in excess to both 210 Pb and 210 Po, with a mean concentration of 1.72±0.03 Bq m. Steady-state balance equations of the three radionuclides permit calculation of residence times for 210 Pb and 210 Po in surface waters (0.8 and 3.0 yr, respectively). 210 Po residence times were almost one order of magnitude higher than mostly reported values. Polonium is known to manifest higher affinity and different binding mechanisms (by entering the organic cycle) to biogenic material than 210 Pb. Several processes, such as 210 Po uptake by buoyant particles and recycling of organic matter in the surface layer, could lead to such long residence times of 210 Po and, therefore, of organic particles in surface waters. Larger 210 Po atmospheric fluxes than those here considered and inputs from rivers and/or fine-grained sediment resuspension could also help to explain the observation, although presumably to a lesser extent.