The total free acids in the juice of mature Valencia and navel oranges were calculated (1) from the titratable acidity, with phenolphthalein as indicator; (2) from the amt. of NaOH required to bring a known volume of orange juice to the equivalence point (pH 7.8 [plus or minus]0.03), when titrating potentiometrically; and (3) from the amt. of NaOH required to neutralize the liberated acetic acid formed when the organic acids and acid salts in the juice are precipitated by Pb acetate. The values detd. by these 3 methods are compared with the actual amt. of citric acid and combined citrates chemically detd. in the various samples. The acid constituents of orarfge juice, as detd. by titrating the juice directly with standard NaOH to phenolphthalein end point, were equivalent to the amts. detd. by the Pb acetate method. The total free acids detd. from the points of inflection on the titration curves were significantly lower than those reported both for the phenolphthalein titration and for the Pb acetate method. When juice samples were titrated, with phenolphthalein as indicator, the end points ranged from pH 8.25 to 8.45. The potentiometric values have been taken to represent more accurately the total acidity. The cone, of malic acid in different juice samples varied only slightly, as compared with the changes in the citric acid content. With one exception, the samples studied had a malic acid content of from 1.4 to 1.77 mg. per ml. of juice, while the citric acid varied from 8.38 to 25.39 mg. per ml. Variations in acidity of the orange are evidently due chiefly to changes in the citric acid cone. Apparently, there is a definite relation between the free acid-combined acid balance and the pH of the juice. The amount of combined acids in the juice is remarkably uniform; this indicates that the free-acid cone, is the chief variable. With a decrease in the value of (total free acid)/(total free and combined acids), a corresponding rise in pH occurs.