An enhanced oil recovery method comprising injecting an aqueous solution of ammonium salts selected from the group consisting of ammonium sulfite, ammonium bisulfite, and mixtures thereof into a petroleum-bearing earth formation, heating said injected aqueous solution to a temperature in the range of about 120.degree.-300.degree. F., or higher in the presence of said petroleum-bearing earth formation, flowing said heated aqueous solution through said petroleum bearing earth formation to drive petroleum to a recovery well, and producing increased amounts of petroleum from said earth formation through said recovery well.