Data on the physico-chemical speciation of Pu in the Irish Sea and the western Mediterranean, gathered in the course of research expeditions carried out in the period 1988-1993, are reviewed in detail in this paper. Measurements of the oxidation state distribution of 239,240Pu (and 238Pu) in filtered water sampled throughout the Irish Sea show little variation, geographically or temporally, with some 87 ± 6% in the oxidized, Pu(V), state overall. No distinction is observed between surface and bottom waters, reflecting both the shallow and the well-mixed nature of these waters. Interestingly, the 241Pu(IV)/239,240Pu(IV) ratio in filtered water from the north-eastern Irish Sea, close to the Sellafield source-term, is found to be significantly higher than the corresponding 241Pu(V)/239,240Pu(V) ratio, while the latter appears to be identical to the 241Pu/239,240Pu ratio in suspended particulate from the same zone. It is suggested that this distinction is of importance in the interpretation of the mechanisms responsible for the hold-up and dispersion of Pu in the near field. The percentage of Pu in colloidal form in open waters, as operationally defined by enhanced sorption on Al2O3, seldom exceeds 15%. There is some evidence of higher percentages in near-shore waters containing proportionately more Pu in the reduced, Pu(IV), state. Chemical speciation and enhanced sorption analyses on samples of ultrafiltered water confirm that a significant proportion of the Pu(IV) is in a colloidal form and that the size of the colloidal particles or aggregates involved (<10 kD) is considerably smaller than that observed in non-saline waters. In contrast to the case of Pu(IV), there is no evidence of significant Pu(V) retention upon ultrafiltration, even by a 1 kD membrane, demonstrating that Pu in an oxidized form is genuinely soluble. The oxidation state distribution of Pu in the western Mediterranean water column was also examined. In open waters, a pronounced sub-surface maximum in the Pu(V) concentration is observed at depths between 250 m and 500 m. Moreover, the percentage of Pu(IV) is found to increase from 5% at a depth of almost 3000 m to a maximum of 70% at about 50 m, most if not all of this increase taking place in the upper half of the water column. Inshore (Gulf of Vera), the percentage of Pu(IV) in surface waters rises to almost 50%, while the percentage in bottom waters remains at 5%. Overall, the bulk of the Pu inventory in the Mediterranean water column is in an oxidized form, particularly in open waters, while the quantities in particulate or colloidal form are comparatively small. © 1995.