Slag heaps at the ancient lead-silver smelting sites of Laurion are of the order of magnitude of several millions of tons. These slags were partly discarded inland (ca. 1.5 million tons) and partly dumped into the sea (probably the same amount). A selection of these were investigated by quantitative XRF and X-ray diffraction. They are characterised by high CaO- and Pb-contents. Their composition varies considerably; mean values are: SiO*l2*s ca. 30 percent, FeO ca. 15 percent, Al*l2*sO*l3*s ca 5 percent, CaO ca. 20 percent and Pb ca. 15 percent. Among the slag minerals identified, hedenbergite, CaFe(Si*l2*sO*l6*s), is the most dominant. Chemical and phase composition of the slags reflect the composition of the ores and the conditions of the smelting process. Argentiferous galena (eventually with minor additions of secondary ores, like cerussite) was submitted to a roasting-reduction- reaction process to obtain argentoferous lead, which was subsequently converted to silver by cupellation. According to Conophagos, the high lead residues in the slags were accepted in order to economise on charcoal during the smelting process (weight ratio of ore to fuel for combustion and reduction ca. 1:0.2). During the main period of activity (5./4. cent. B.C.), the Laurion smelters are said to have produced ca. 1.4 million tons of lead and about 3500 tons of silver.