A combination of laboratory studies using synthetic annite, thermodynamic modeling, and analysis of compositions of natural biotities suggests that the equilibrium constant for the exchange reaction Bi-OH + HCl = Bi-Cl + H 2 O can be expressed as log K(OH-Cl) = 5151/T(K) - 5.01 - 1.98 X Mg , using for standard states the pure ideal gas H 2 O and HCl at 1 bar and T, pure hydroxyl-biotite, and (hypothetical) chlorbiotite at P and T and X Mg as the mole fraction of octahedral Mg in biotite. The exchange reaction was directly measured in the Mg-free system at 1 kb and 445 degrees to 575 degrees C using a synthetic biotite of nominal annite composition. The assemblage muscovite-quartz-K-feldspar-H 2 O-HCl (2.0 m total Cl) buffered the fugacity ratio HCl/H 2 O in the fluid phase. Analogous experiments using phlogopites were unsuccessful because of the small amount of Cl accepted (less than 0.1%), as compared with annite (up to 1.4%). Accordingly, the quantitative effect of Mg [harr] Fe substitution on Cl [harr] OH exchange was evaluated using chemical data from a suite of biotites from the porphyry copper deposit at Santa Rita, New Mexico (Jacobs, 1976). When combined with analogous data for F [harr] OH exchange, the equilibrium constant for the halogen exchange reaction Bi-Cl + HF = Bi-F + HCl is: log K(F-Cl) = 3051/T - 5.34 + 3.13 X Mg (see note). This equation is used to compare the Santa Rita biotites with biotites from the porphyry molybdenum deposit at Henderson, Colorado (Gunow, 1978). These two biotite suites are strongly separated on a log (X F /X Cl - X Mg plot and suggest a difference in fugacity ratio of HF to HCl in their respective hydrothermal fluids of more than an order of magnitude, with the Henderson fluids being the more fluorine rich.