The fate of bare and polymer coated-AgNP was studied in river and coastal waters and factors influencing dissolution kinetics such as polymer coating, dissolved oxygen content and ionic composition have been explored. As expected, bare- AgNP showed slow aggregation process in river water (RW), but seawater (SW) induced a fast auto-aggregation due to high ionic strength. Due to strong repulsive forces of amine groups, poly(allyl)amine coated nanoparticles (PAAm- AgNP) showed only a slight aggregation in RW and SW. Both bare- and PAAm- AgNP released immediately after dispersion a notable amount of free Ag, a result attributed to a fast initial dissolution of surface Ag2O for bare-AgNP and to Ag+ displacement from coating polymer by competing cations for PAAm- AgNP. Dissolution in river water over two weeks was not evidenced for both bare- and PAAm-AgNP, but a slow and constant increase of free Ag (as soluble chloro-complexes) was observed for PAAm-AgNP in SW. A low oxygen content (<25% saturation) did not markedly affect aggregation and dissolution for both AgNP species in RW and SW. Results suggest a very slow dissolution of both bareand polymer coated-AgNP in river water where natural dissolved organic matter may have play a role. Taking in account their high stability, very low solubility and hetero-aggregation with suspended particulate matter, these nanoparticles would be persistent and potentially transported to surface coastal waters in days and weeks following their discharge where they are expected to slowly dissolve and be available to marine organisms.