Lead slags from two different metallurgical technologies (primary slag from Pb-ore processing and secondary slag from car battery recycling) were submitted to 12-year batch leaching at a L/S ratio of 10 and the leaching data were compared with the previously obtained results from a ⩽1-year leaching test. In the long-term, the Pb release is efficiently controlled by the precipitation of cerussite (PbCO3), Pb-phosphates and anglesite (PbSO4) and by sorption onto newly formed hydrous ferric oxides. In contrast, Zn is the most important contaminant that is continuously released from the primary slag and, after leaching for 12years, its concentration exceeds more than 40× the regulatory limit value for hazardous wastes (200mg/kg). For this reason, the recycling of primary slag for civil engineering purposes (concrete technology, gravel-like material) cannot be recommended, because of the long-term Zn leaching. In terms of the leaching of metals (Pb, Zn, Cu) and metalloids (As, Sb), the secondary slag mostly fulfils the limit criteria for inert waste according to the EU legislation even for long-term leaching. However, high release of Ba exceeding more than 10× the limit values for hazardous waste was observed, despite the massive formation of barite (BaSO4) on the slag surface. These results indicate that less common contaminants (e.g. Ba), which have been overlooked so far, should also be considered during assessment of the leaching behaviour of smelting slags with a view to their safe recycling in civil engineering.