The solubility of the buffer assemblage magnetite+hematite in 0.10, 1.00 and 3.00 mol/kg NaCl solutions with various pH values was determined by use of a Dickson gold cell hydrotherma1 apparatus at 300℃ and 500 bars. Activities of H+ and Cl-in the NaCl hydrothermal solution for each determination were approximated. The experimental results show that the solubility is strongly dependent on the pH of the solutions and appears to have no explicit relation to NaCl molality in acidic solutions. The correlation of the measured solubility with pH at 300℃ suggests a change in Fe speciation in solution at pH≈5.3-5.4. Treatment of the experimental data by a ridge regressions program and an iterative procedure indicates that the dominant species in solution under the run condition are FeOH+, Fe(OH)3Cl-, Fe(OH)03 and Fe(OH)-4. Gibbs free energy of formation for these species at 300℃ and 500 bars were derived: 0ΔGf,FeOH+=-250.4 kJ/mol; ΔG0f,Fe(OH)3Cl−=-644.7 kJ/mol; ΔG0f,FeOH03=-552.6 kJ/mol; and ΔG0f,FeOH-4=-652.7 kJ/mol. The Present study demonstrates that the change in acidity of hydrothermal ore-forming solutions exerts an important influence on the transport and deposition of Fe in nature.