A total of 268 thermal spring samples were analyzed for total soluble As using reduced molybdenum-blue; 27 of these samples were also analyzed for total Sb using flame atomic absorption spectrometry. At Yellowstone the ClAs atomic ratio is nearly constant among neutral-alkaline springs with Cl > 100 mg L⁻¹, and within restricted geographic areas, indicating no differential effects of adiabatic vs. conductive cooling on arsenic. The ClAs ratio increases with silica and decreases with decreasing ClΣCO3; the latter relationship is best exemplified for springs along the extensively sampled SE-NW trend within the Lone Star-Upper-Midway Basin region. The relationship between ClAs and ClΣCO3 at Yellowstone suggests a possible rock leaching rather than magmatic origin for much of the Park's total As flux. Condensed vapor springs are low in both As and Cl. Very high ClAs ratios ( > 1000) are associated exclusively with highly diluted (Cl < 100 mg L⁻¹) mixed springs in the Norris and Shoshone Basins and in the Upper White Creek and Firehole Lake areas of Lower Basin. The high ratios are associated with acidity and/or oxygen and iron; they indicate precipitation of As following massive dilution of the Asbearing high-Cl parent water.

Yellowstone Sb ranged from 0.009 at Mammoth to 0.166 mg L⁻¹ at Joseph's Coat Spring. Within basins, the ClSb ratio increases as the ClΣCO3 ratio decreases, in marked contrast to As. Mixed springs also have elevated ClSb ratios. White (1967) and Weissberg (1969) previously reported stibnite (Sb₂S₃), but not orpiment (As₂S₃), precipitating in the near surface zone of alkaline geothermal systems.