The flat oyster, Ostrea edulis, was consumed as a luxury dish by the Romans in antiquity. Numerous shells are found in archaeological sites in the Lyon region, Central France. This area is located over 250 km away from the nearest coastline (the Mediterranean Sea) and little is known about the origin of these oysters prior to transport for consumption. The chemistry of biogenic carbonates reflects that of the fluid they precipitate from at the time of formation. Stable isotopes and Mg/Ca ratios in oyster shells have previously been used as palaeoenvironmental proxies. As Mg/Ca ratio amplitude in bivalve shells has been reported to differ according to local hydrologic settings, we suggest that geochemical differences observed in each shell can be used to identify the type of environment (e.g. estuary, lagoon or marine) from which the specimens originated.