Solubilities of magnesium sulphate in sulphuric acid solutions and hydrolysis of acidic ferric sulphate solutions of various concentrations with the addition of magnesium sulphate have been investigated in an autoclave at temperatures of 185°C and 200°C.<BR>The results obtained were as follows:<BR>1. Solubilities of magnesium sulphate increased with an increase in sulphuric acid concentrations in solutions, while at the same sulphuric acid concentration, solubilities of magnesium sulphate at 185°C were slightly higher than that at 200°C.<BR>2. Solubilities of magnesium sulphate in sulphuric acid solutions at temperatures of 185°C and 200°C were represented by the following emnirical formulas:<BR>Mg²⁺ (g/l)=4.981+3.044×10^-1 [H₂SO₄ (g/l)]-4.651×0^-1 [H₂SO⁴ (g/l)] ²at a tenlperature of 185°C<BR>MgSUP>²⁺ (g/l)=2.962+2.947×10-^-1 [H₂SO₄ (g/l)]-3.889×10^-4 [HH₂SO⁴ (g/l)] ² at a temperature of 200°C<BR>3. When the hydrolysis of ferric sulphate soulutions was at equilibrium, the concentration of ferric ion increased with an increase in that of sulphuric acid in solutions, while at the same sulphuric acid concentration, the concentration of ferric ion decreased considerably with an increase in the addition of magnesium sulphate.<BR>4. In the hydrolysis of acidic ferric sulphate solutions, ferric oxide, Fe₂0₃, was precipitated at lower sulphuric acid concentrations, and ferric basic sulphate, Fe (OH) SO₄, was precipitated at higher concentrations of sulphuric acid, while hydrolysis and precipitation of ferric oxide increased withthe addition of magnesium sulphate in the ferric sulphate solutions.<BR>5. At a temperature of 185°C in the absence of magnesium sulphate, hydrolysis and precipitation of ferric oxide occurred up to the sulphuric acid concentration of 55.8g/l or below, and at 55.8g/l or above, ferric basic sulphate was precipitated. When respectively about 15g/l Mg²⁺ion and 30g/l Mg²⁺ion as magnesium sulphate were added in the ferric sulphate solutions, hydrolysis and precipitation of ferric oxide occurred up to the sulphuric acid concentration of 76.5g/l and 100.5g/l.<BR>6. At a temperature of 200°C in the absence of magnesium sulphate, hydrolysis and precipitation of ferric oxide occurred up to the sulphuric acid concentration of 69.3g/l or below, and at 69.3g/l or above, ferric basic sulphate was precipitated. When respectively about 15g/l Mg²⁺ion and 30g/lMg²⁺ion as magnesium sulphate were added, hydrolysis and precipitation of ferric oxide occurred up to the sulphuric acid concentration of 82.1g/l and 105.1g/l.<BR>7. When magnesium sulphate was added in solutions, empirical formulas of the ferric ion concentration and the sulphuric acid concentration over the stable region of ferric oxide were as follows. at a temperature of 185°C<BR>log [Fe³⁺ (g/l)] =5.30 log [H₂SO₄ (g/l)]-9.61 about 15g/l Mg²⁺ion as magnesium sulphate log [Fe³⁺ (g/l)] =5.46 log [H₂SO₄ (g/l)] -10.24 about 30g/l Mg²⁺ion as magnesium sulphate at a temperature of 200°C<BR>log [Fe³⁺ (g/l)] =5.09 log [H₂SO₄ (g/l)] -9.46 about 15g/l Mg²⁺ion as rnagnesium sulphate log [Fe³⁺ (g/l)] =5.74 log [H₂SO₄ (g/l)] -11.02 about 30g/l Mg²⁺ion as magnesium sulphate