Molten calcium halide and hydroxide salts may be used as flux and reactants in determining apatite exchange equilibria and solid solution behavior. New experimental data and published phase equilibria are used to determine a thermodynamic model of melts and solids along the binary joins of the anhydrous apatite system Ca 3(PO 4) 2CaCl 2CaF 2Ca(OH) 2. In this model, melt components are expressed as Ca0.5X, where X is hydroxide or a halogen, or as Ca 1.5PO 4. The derived binary interaction parameters ( WG' s) are sufficient to describe deviations from Raoultian behavior for the data. Standard state data are derived for molten Ca(OH) 2 and Ca 3(PO 4) 2, and for the intermediate compounds CaClF, CaClOH, and Ca 2PO 4Cl. Melts in the CaCl 2CaF 2 system show ideal mixing behavior, while the calcium hydroxide-bearing molten salts form asymmetric regular solutions. The hydroxide-bearing melts show the small positive enthalpies of mixing typical for mixed-anion salts. Similar behavior is measured in simpler molten salt mixtures. The positive deviations from Raoultian behavior are not a mathematical artifact of the model. Data are less extensive for the salt-phosphate systems but cover the essential portions of the systems where salt to phosphate ratios are high. The mixtures of the molten salts and phosphates salts show large negative enthalpy interaction parameters ( WH), as is expected in eutectic systems which mix a high-melting-point crystal with a low-melting-point flux. Adequate reproduction of the data requires that some entropy interaction parameters be negative, although small. This implies the presence of ordering in the melt, which is manifested in more polymerized phosphate liquids and glasses as halogenated and hydroxylated orthophosphate and pyrophosphate species. Results of the model indicate that the quaternary system is a good choice for determination of activity-composition relations for the apatites. Comparison of common sources of standard state thermodynamic data ( ROBIE et al., 1979; JANAF, CHASE et al., 1985; CODATA, GARVIN et al., 1987) shows that the data for the salts are very similar where they are not identical. At temperatures below 900°C in the ternary systems, liquid compositions will be on or near the apatite-crystalline salt cotectics, and dissolve less than 1 mol% Ca 3(PO 4) 2. This makes CaCl 2CaF 2 mixtures excellent candidates for determination of unambiguous apatite activity-compositional relations. Only the Ca(OH) 2 liquidus is strongly affected by the addition of phosphate, but this effect is closely described by a regular solution model.