Observations of halite and sylvite disappearance temperatures in fluid inclusions from high-temperature deposits indicate that the trapped solutions have compositions lying on linear trends in the NaCl-KCl-H 2 O system. These trends were formed by crystallization of KCl-bearing halite from the hydrothermal solutions before the liquid was trapped in fluid inclusions. Consideration of phase equilibria in the NaCl-KCl-H 2 O system indicates that the required pre-trapping, halite-liquid equilibria could be caused by reaction paths such as: (1) the EL path, in which halite separates from liquids under sufficient pressure to avoid the presence of vapor, i.e., if pressure decreases sufficiently prior to trapping, a vapor phase will form subsequently and the inclusions will homogenize by vapor disappearance but fall on the halite trend; (2) the EV path, in which a vapor phase crystallizes halite and then separates from it and partially condenses to liquid, which in turn becomes separated and trapped in inclusions without further compositional change; (3) the DL path, in which a decrease in pressure places the system into the vapor + halite field, thus causing the liquid to boil and deposit halite; and (4) the SC path, in which liquids are trapped while on the solubility surface in the NaCl-KCl-H 2 O system (in equilibrium with halite and vapor) or after cooling (with little pressure drop) to yield the assemblage liquid + halite.Solutions trapped while undergoing each of these processes will yield a specific, and in some cases distinctive, assemblage of inclusion types, thus permitting designation of the probable path that formed the observed halite trend. This, in turn, permits limits to be put on the pressures and temperatures of mineralization. Experimental data on Na, K, and Ca contents of fluids in equilibrium with granitic rocks as well as data on the partitioning of Cl between magma and aqueous phases support the interpretation that solutions at the high-temperature end of the halite trend originated by exsolution from a granitic magma.