Oaxacan urns, the most characteristic esthetic expression of Zapotec culture in ancient Mesoamerica, are an important potential source of information for archaeologists and art historians, but relatively few exist with specific archaeological context and many are suspect as forgeries. This paper reports the thermoluminescent authenticity determinations of over one hundred Oaxacan urns as a first step toward confirmining a sequence previously determined by stylistic analysis. We tested a collection of 117 urns on loan to the St Louis Art Museum, mostly acquired after 1960, and another group of 6 from the Peabody Museum, Harvard, acquired before 1930. Our hypothesis that more recent collections would contain fewer forgeries was confirmed and a number of stylistic criteria for determining authenticity are critically examined in the light of the results.