Herodotus (5th century B.C.) and Pausanias (A.D. 2nd century) mentioned that silver-and gold-rich mines were the cause of the prosperity of Siphnos in the sixth century B.C. The work site at Ayos Sostis is traditionally identified with those mines. Ore and slag samples from the site were studied as part of a project aiming at tracing the silver sources for ancient Greek coins. The samples were studied by reflection microscopy, and electron microprobe analysis. Argentite (Ag₂S) grains up to 15mu were found in weathered lead and copper ores and in galena. No evidence for gold was found. Studies of slags and ores showed that copper and silver extraction and purification were performed at the site. The analysis of copper inclusions in the slag product of careless extraction (1.5 percent by weight of nickel, 0.6 percent by weight of antimony, and 4. 15 percent by weight of arsenic) suggests that the copper extracted at the site was alloyed with the silver used in the minting of some Aeginetan coins.