The chemical analysis of a residue inside a Late Uruk Period (c. 3500-3100 BC) spouted jar from Warka (ancient Uruk) produced positive evidence for the presence of tartaric acid. The shape and other features of this jar type, as well as the nature of the residue, indicate that it originally contained a liquid, and was probably a serving vessel for a beverage. Tartaric acid occurs in large amounts in nature only in grapes. Under normal conditions and at room temperature, grape juice easily and quickly ferments to wine. This finding supports Algaze’s recent suggestion that the widespread occurrence of this jar type at Late Uruk Period sites, especially in upland regions, is related to a wine trade. The presence of a grape residue within the Warka jar suggests that the function of similar jars found elsewhere was also to serve wine, although any customs or circumstances surrounding this activity, whether local or southern Mesopotamian, is not known. Since grapevines are not native to the region around Warka and grow with difficulty in Lower Mesopotamia, it is likely the contents of this vessel were imported.