This paper focuses on an Athenian decree implementing a fundamental metrological reform at the end of the 2nd century BCE. This text has been known for a long time, since it was seen and copied by Michel Fourmont in Athens in 1729. The inscription is now lost, so that Fourmont’s sketch is our main source for this decree, together with a small fragment of a copy of the same decree found during the American excavations of the Agora in 1934. However, all current editions, including CIG I 123 (Boeckh, 1828), IG II 476 (Koehler, 1877), and IG II–III2 1013 (Kirchner, 1916), are not based on the draft sketch drawn by Michel Fourmont in Athens in 1729, but on a final version made by Claude-Louis Fourmont—Michel’s nephew—in Paris, probably between 1742 and 1746. Since the final version significantly differed from the original, it was necessary to re-edit the Athenian decree on the basis of the draft version (Supplément grec 854, fol. 79r–81r, no. 142), reject the supplements suggested by the previous editors, modify several restorations and interpretations, and revise the text structure. This paper also summarizes the main data provided by this decree about the Athenian metrological system during the late Hellenistic period, and challenges the communis opinio that the Athenians adopted the Roman standards at the end of the 2nd century BCE.