Abstract Settling particles and underlying sediments collected at 1, 2.5 and 4 km along offshore transects in the urbanized sector of the Río de la Plata were analyzed to evaluate the sources and accumulation of PCBs. Total PCB concentrations range from <0.1 to 100 ng g −1 and include variability associated to North–South and offshore gradients reflecting the impact of coastal discharges. Highest concentrations were recorded in the industrialized Central area close to Buenos Aires (61 ± 37 ng g −1 at 1 km) relative to cleaner northern stations (3.6 ± 2.2 ng g −1) and southward sites (37 ± 2.8 ng g −1), affected by transport of particulate PCBs by coastal currents. Sediment traps deployed in the Central area revealed large depositional fluxes of total matter (361 ± 124 g m −2 day −1) and PCBs (26 ± 19 μg m −2 day −1) and high sedimentation rates (5.0 ± 1.7 cm yr −1). Uniform PCB concentrations (66–89 ng g −1) down to 20 cm in sediment cores suggest continued PCB discharges during the last 4 years. PCB composition was dominated by hexa (43 ± 6.4%), hepta (23 ± 5.1%) and pentachlorobiphenyls (21 ± 5.5%) with lower proportions tri–tetra (7.4 ± 5.4%) and higher chlorinated congeners (5.1 ± 3.3%). A consistent weathering pattern with loss of 3–5 chlorobiphenyls and enrichment in higher chlorinated PCBs corresponding to a shift from a 1:1 to a 1:3 1254:1260 Aroclor mixture, was observed offshore. A principal component analysis performed with the relative contribution of PCB congener classes confirmed the offshore weathering pattern indicating that transformer oils containing Aroclor 1254–1260 are the most probable sources. Sediment inventories, sediment trap fluxes and Fugacity II calculations indicate an accumulation ∼500–800 kg PCB in superficial sediments of this coastal environment.