The Wilson equation (Wilson 1964) is applied to (Fe,Mn,Mg)TiO 3 solid solutions for obtaining the mixing properties of the ternary solid solution at 600 degrees C and 1 kbar. The present study utilizes data on cation exchange between (Fe,Mn)Cl (sub 2(aq)) and (Fe,Mn)TiO 3 , between (Mn,Mg)Cl (sub 2(aq)) and (Mn,Mg)TiO 3 , and between (Fe,Mg)Cl (sub 2(aq)) and (Fe,Mg)TiO 3 (Kubo et al. 1992). The molar excess Gibbs energy (G ex ) is the following: G ex (kJ/mol) = -7.260[X FeTiO3 ln(X FeTiO3 + 1.314X MnTiO3 + 0.962X MgTiO3 ) + X MnTiO3 ln(0.585X FeTiO3 + X MnTiO3 + 0.393X MgTiO3 ) + X MgTiO3 ln(0.406X FeTiO3 + 0.371X MnTiO3 + X MgTiO3 )], where X stands for the mole fraction of the subscripted component. The predicted compositions of (Fe,Mn,Mg)Cl (sub 2(aq)) fluids in equilibrium with the ternary solid solutions are in good agreement with the experimental values.