What was the role of expert chemists in evaluating medical cures in early modern Europe? The case of mineral waters in France is a rich source of answers to this question. Mineral waters were the subject of prolonged and manifold researches by members of the Académie Royale des Sciences, the main source of learned chemical knowledge in France the period. The Academy began analyzing and evaluating mineral springs soon after it was founded in 1666. The academicians thereby took on a role that had previously been the sole preserve of practicing physicians. In this project, I examine the effect that the increasingly sophisticated analyses done by academic chemists had on the way that non-academic physicians appraised mineral waters, and on the way that physicians and spa owners resolved disputes about the medical benefits of mineral waters. I find that physicians started studying the chemical composition of mineral waters, and not just their observed effects on patients, well before 1666. The academicians’ analyses nevertheless had a significant impact on the way waters were evaluated and on the fortunes of individuals spas—especially those at Passy, whose popularity from the 1720s onwards owed much to academic chemists.