Theoretical aspects are presented of a technique for modeling the interrelationships between variations in f (sub O 2 ) , f (sub S 2 ) , f (sub H 2 O) , temperature, and pressure and the transitions in mineral assemblages and chemistries of oxides, sulfides, and silicates associated with metamorphosed ore deposits. The basic approach utilized is to evaluate the controls on the distribution of iron between oxides, sulfides, and silicates. The stabilities of Fe-bearing silicates are evaluated as functions of a FeO and a (sub Al 2 SiO 5 ) (assuming a (sub SiO 2 ) = 1, a (sub H 2 O) = 1) in conjunction with constraints provided by Fe-Mg K D equations. The resulting equations are used to predict variations in silicate assemblages and X Fe 's of Fe-bearing silicates with varying a FeO and/or a (sub Al 2 SiO 5 ) .Using equations relating a FeO to f (sub O 2 ) in the presence of an Fe oxide, it is possible to relate the calculated silicate equilibria to variations in f (sub O 2 ) , or if both an Fe oxide and an Fe sulfide are present, f (sub O 2 ) /f (sub S 2 ) . The equations are used to construct f (sub O 2 ) or f (sub O 2 ) /f (sub S 2 ) versus X Fe diagrams for evaluation of the effects of f (sub O 2 ) and/or f (sub S 2 ) on oxide-sulfide-silicate assemblages and chemistries. The results are employed also in the construction of calibrated AFM and f (sub O 2 ) - f (sub S 2 ) diagrams. The equilibria can be calibrated either from tabulated thermodynamic data or by extrapolations from experimental results.