Possible mechanisms for the transportation of quicksilver in vein fluids are examined by the methods of chemical thermodynamics. Mechanisms shown to be impossible are those depending on decrease of ionization or solubility of H 2 S at high temperatures, on elimination of H 2 S at high temperatures by displacement of equilibrium in the gas phase, and on the formation of complex mercury-halogen complex ions. Mechanisms shown to be feasible are transportation as HgS 2 -- in alkaline sulfide solutions, as the volatile chloride in the vapor phase, and as the vapor of the element itself. Data are conflicting about the possibility of transportation in acid or neutral solutions as a sulfomercuric acid. Transportation as the sulfide in supersaturated solutions or as a colloidal dispersion seems possible, but cannot be quantitatively evaluated from present data.A reasonable explanation for the separation of cinnabar deposits from deposits of most other sulfide ores is the thermal instability of mercuric sulfide. At temperatures over 250 degrees C the sulfide cannot exist in the presence of a moving vapor, unless concentrations of mercury and sulfur vapors are improbably high. Even at temperatures well below this point mercury and sulfur can be carried in appreciable concentrations in a moving vapor.