We report the results of an isotopic study designed to determine the source of solutes and carbon dioxide in the famed Saratoga Springs (New York) mineral waters. These waters have thousands of milligrams per liter total dissolved solid concentrations and are highly charged with carbon dioxide gas. The spring waters are cold (∼12 °C) and there is no local, deep-seated thermal anomaly. They emerge through thick shale caprock along the surface expression of normal faults. The δ13C (−5.8‰ to +0.8‰ Vienna Peedee belemnite) of the dissolved inorganic carbon and elevated 3He/4He ratios suggest that the source of the CO2 is the mantle or an ancient deep crystallized igneous melt. The stable isotopic content of the spring waters defines a mixing line between modern local meteoric waters (δ ∼ 70‰) and a component with heavier δD but similar δ18O values. This trend and that of 87Sr/86Sr of dissolved strontium versus 1/Sr are consistent with the hypothesis that Canadian Shield–type brines contribute salinity to the springs. These brines plausibly migrate from the Adirondack Mountains to the topographically low McGregor fault system in the Hudson River lowlands, where the Saratoga springs discharge.